/*
 * %W% %E%
 *
 * Copyright (c) 2006, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */

package java.net;

import java.io.IOException;
import java.io.InvalidObjectException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.nio.ByteBuffer;
import java.nio.CharBuffer;
import java.nio.charset.CharsetDecoder;
import java.nio.charset.CharsetEncoder;
import java.nio.charset.CoderResult;
import java.nio.charset.CodingErrorAction;
import java.nio.charset.CharacterCodingException;
import java.text.Normalizer;
import sun.nio.cs.ThreadLocalCoders;

import java.lang.Character;		// for javadoc
import java.lang.NullPointerException;	// for javadoc


/**
 * Represents a Uniform Resource Identifier (URI) reference.
 *
 * <p> Aside from some minor deviations noted below, an instance of this 
 * class represents a URI reference as defined by
 * <a href="http://www.ietf.org/rfc/rfc2396.txt""><i>RFC&nbsp;2396: Uniform
 * Resource Identifiers (URI): Generic Syntax</i></a>, amended by <a
 * href="http://www.ietf.org/rfc/rfc2732.txt"><i>RFC&nbsp;2732: Format for
 * Literal IPv6 Addresses in URLs</i></a>. The Literal IPv6 address format
 * also supports scope_ids. The syntax and usage of scope_ids is described
 * <a href="Inet6Address.html#scoped">here</a>.
 * This class provides constructors for creating URI instances from
 * their components or by parsing their string forms, methods for accessing the
 * various components of an instance, and methods for normalizing, resolving,
 * and relativizing URI instances.  Instances of this class are immutable.
 *
 *
 * <h4> URI syntax and components </h4>
 *
 * At the highest level a URI reference (hereinafter simply "URI") in string
 * form has the syntax
 *
 * <blockquote>
 * [<i>scheme</i><tt><b>:</b></tt><i></i>]<i>scheme-specific-part</i>[<tt><b>#</b></tt><i>fragment</i>]
 * </blockquote>
 *
 * where square brackets [...] delineate optional components and the characters
 * <tt><b>:</b></tt> and <tt><b>#</b></tt> stand for themselves.
 *
 * <p> An <i>absolute</i> URI specifies a scheme; a URI that is not absolute is
 * said to be <i>relative</i>.  URIs are also classified according to whether
 * they are <i>opaque</i> or <i>hierarchical</i>.
 *
 * <p> An <i>opaque</i> URI is an absolute URI whose scheme-specific part does
 * not begin with a slash character (<tt>'/'</tt>).  Opaque URIs are not
 * subject to further parsing.  Some examples of opaque URIs are:
 *
 * <blockquote><table cellpadding=0 cellspacing=0 summary="layout">
 * <tr><td><tt>mailto:java-net@java.sun.com</tt><td></tr>
 * <tr><td><tt>news:comp.lang.java</tt><td></tr>
 * <tr><td><tt>urn:isbn:096139210x</tt></td></tr>
 * </table></blockquote>
 *
 * <p> A <i>hierarchical</i> URI is either an absolute URI whose
 * scheme-specific part begins with a slash character, or a relative URI, that
 * is, a URI that does not specify a scheme.  Some examples of hierarchical
 * URIs are:
 *
 * <blockquote>
 * <tt>http://java.sun.com/j2se/1.3/</tt><br>
 * <tt>docs/guide/collections/designfaq.html#28</tt><br>
 * <tt>../../../demo/jfc/SwingSet2/src/SwingSet2.java</tt><br>
 * <tt>file:///~/calendar</tt>
 * </blockquote>
 *
 * <p> A hierarchical URI is subject to further parsing according to the syntax
 *
 * <blockquote>
 * [<i>scheme</i><tt><b>:</b></tt>][<tt><b>//</b></tt><i>authority</i>][<i>path</i>][<tt><b>?</b></tt><i>query</i>][<tt><b>#</b></tt><i>fragment</i>]
 * </blockquote>
 *
 * where the characters <tt><b>:</b></tt>, <tt><b>/</b></tt>,
 * <tt><b>?</b></tt>, and <tt><b>#</b></tt> stand for themselves.  The
 * scheme-specific part of a hierarchical URI consists of the characters
 * between the scheme and fragment components.
 *
 * <p> The authority component of a hierarchical URI is, if specified, either
 * <i>server-based</i> or <i>registry-based</i>.  A server-based authority
 * parses according to the familiar syntax
 *
 * <blockquote>
 * [<i>user-info</i><tt><b>@</b></tt>]<i>host</i>[<tt><b>:</b></tt><i>port</i>]
 * </blockquote>
 *
 * where the characters <tt><b>@</b></tt> and <tt><b>:</b></tt> stand for
 * themselves.  Nearly all URI schemes currently in use are server-based.  An
 * authority component that does not parse in this way is considered to be
 * registry-based.
 *
 * <p> The path component of a hierarchical URI is itself said to be absolute
 * if it begins with a slash character (<tt>'/'</tt>); otherwise it is
 * relative.  The path of a hierarchical URI that is either absolute or
 * specifies an authority is always absolute.
 *
 * <p> All told, then, a URI instance has the following nine components:
 *
 * <blockquote><table summary="Describes the components of a URI:scheme,scheme-specific-part,authority,user-info,host,port,path,query,fragment">
 * <tr><th><i>Component</i></th><th><i>Type</i></th></tr>
 * <tr><td>scheme</td><td><tt>String</tt></td></tr>
 * <tr><td>scheme-specific-part&nbsp;&nbsp;&nbsp;&nbsp;</td><td><tt>String</tt></td></tr>
 * <tr><td>authority</td><td><tt>String</tt></td></tr>
 * <tr><td>user-info</td><td><tt>String</tt></td></tr>
 * <tr><td>host</td><td><tt>String</tt></td></tr>
 * <tr><td>port</td><td><tt>int</tt></td></tr>
 * <tr><td>path</td><td><tt>String</tt></td></tr>
 * <tr><td>query</td><td><tt>String</tt></td></tr>
 * <tr><td>fragment</td><td><tt>String</tt></td></tr>
 * </table></blockquote>
 *
 * In a given instance any particular component is either <i>undefined</i> or
 * <i>defined</i> with a distinct value.  Undefined string components are
 * represented by <tt>null</tt>, while undefined integer components are
 * represented by <tt>-1</tt>.  A string component may be defined to have the
 * empty string as its value; this is not equivalent to that component being
 * undefined.
 *
 * <p> Whether a particular component is or is not defined in an instance
 * depends upon the type of the URI being represented.  An absolute URI has a
 * scheme component.  An opaque URI has a scheme, a scheme-specific part, and
 * possibly a fragment, but has no other components.  A hierarchical URI always
 * has a path (though it may be empty) and a scheme-specific-part (which at
 * least contains the path), and may have any of the other components.  If the
 * authority component is present and is server-based then the host component
 * will be defined and the user-information and port components may be defined.
 *
 *
 * <h4> Operations on URI instances </h4>
 *
 * The key operations supported by this class are those of
 * <i>normalization</i>, <i>resolution</i>, and <i>relativization</i>.
 *
 * <p> <i>Normalization</i> is the process of removing unnecessary <tt>"."</tt>
 * and <tt>".."</tt> segments from the path component of a hierarchical URI.
 * Each <tt>"."</tt> segment is simply removed.  A <tt>".."</tt> segment is
 * removed only if it is preceded by a non-<tt>".."</tt> segment.
 * Normalization has no effect upon opaque URIs.
 *
 * <p> <i>Resolution</i> is the process of resolving one URI against another,
 * <i>base</i> URI.  The resulting URI is constructed from components of both
 * URIs in the manner specified by RFC&nbsp;2396, taking components from the
 * base URI for those not specified in the original.  For hierarchical URIs,
 * the path of the original is resolved against the path of the base and then
 * normalized.  The result, for example, of resolving
 *
 * <blockquote>
 * <tt>docs/guide/collections/designfaq.html#28&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</tt>(1)
 * </blockquote>
 *
 * against the base URI <tt>http://java.sun.com/j2se/1.3/</tt> is the result
 * URI
 *
 * <blockquote>
 * <tt>http://java.sun.com/j2se/1.3/docs/guide/collections/designfaq.html#28</tt>
 * </blockquote>
 *
 * Resolving the relative URI
 *
 * <blockquote>
 * <tt>../../../demo/jfc/SwingSet2/src/SwingSet2.java&nbsp;&nbsp;&nbsp;&nbsp;</tt>(2)
 * </blockquote>
 *
 * against this result yields, in turn,
 *
 * <blockquote>
 * <tt>http://java.sun.com/j2se/1.3/demo/jfc/SwingSet2/src/SwingSet2.java</tt>
 * </blockquote>
 *
 * Resolution of both absolute and relative URIs, and of both absolute and
 * relative paths in the case of hierarchical URIs, is supported.  Resolving
 * the URI <tt>file:///~calendar</tt> against any other URI simply yields the
 * original URI, since it is absolute.  Resolving the relative URI (2) above
 * against the relative base URI (1) yields the normalized, but still relative,
 * URI
 *
 * <blockquote>
 * <tt>demo/jfc/SwingSet2/src/SwingSet2.java</tt>
 * </blockquote>
 *
 * <p> <i>Relativization</i>, finally, is the inverse of resolution: For any
 * two normalized URIs <i>u</i> and&nbsp;<i>v</i>,
 *
 * <blockquote>
 *   <i>u</i><tt>.relativize(</tt><i>u</i><tt>.resolve(</tt><i>v</i><tt>)).equals(</tt><i>v</i><tt>)</tt>&nbsp;&nbsp;and<br>
 *   <i>u</i><tt>.resolve(</tt><i>u</i><tt>.relativize(</tt><i>v</i><tt>)).equals(</tt><i>v</i><tt>)</tt>&nbsp;&nbsp;.<br>
 * </blockquote>
 *
 * This operation is often useful when constructing a document containing URIs
 * that must be made relative to the base URI of the document wherever
 * possible.  For example, relativizing the URI
 *
 * <blockquote>
 * <tt>http://java.sun.com/j2se/1.3/docs/guide/index.html</tt>
 * </blockquote>
 *
 * against the base URI
 *
 * <blockquote>
 * <tt>http://java.sun.com/j2se/1.3</tt>
 * </blockquote>
 *
 * yields the relative URI <tt>docs/guide/index.html</tt>.
 *
 *
 * <h4> Character categories </h4>
 *
 * RFC&nbsp;2396 specifies precisely which characters are permitted in the
 * various components of a URI reference.  The following categories, most of
 * which are taken from that specification, are used below to describe these
 * constraints:
 *
 * <blockquote><table cellspacing=2 summary="Describes categories alpha,digit,alphanum,unreserved,punct,reserved,escaped,and other">
 *   <tr><th valign=top><i>alpha</i></th>
 *       <td>The US-ASCII alphabetic characters,
 * 	  <tt>'A'</tt>&nbsp;through&nbsp;<tt>'Z'</tt>
 * 	  and <tt>'a'</tt>&nbsp;through&nbsp;<tt>'z'</tt></td></tr>
 *   <tr><th valign=top><i>digit</i></th>
 *       <td>The US-ASCII decimal digit characters,
 *       <tt>'0'</tt>&nbsp;through&nbsp;<tt>'9'</tt></td></tr>
 *   <tr><th valign=top><i>alphanum</i></th>
 *       <td>All <i>alpha</i> and <i>digit</i> characters</td></tr>
 *   <tr><th valign=top><i>unreserved</i>&nbsp;&nbsp;&nbsp;&nbsp;</th>
 *       <td>All <i>alphanum</i> characters together with those in the string
 * 	  <tt>"_-!.~'()*"</tt></td></tr>
 *   <tr><th valign=top><i>punct</i></th>
 *       <td>The characters in the string <tt>",;:$&+="</tt></td></tr>
 *   <tr><th valign=top><i>reserved</i></th>
 *       <td>All <i>punct</i> characters together with those in the string
 * 	  <tt>"?/[]@"</tt></td></tr>
 *   <tr><th valign=top><i>escaped</i></th>
 *       <td>Escaped octets, that is, triplets consisting of the percent
 *           character (<tt>'%'</tt>) followed by two hexadecimal digits
 *           (<tt>'0'</tt>-<tt>'9'</tt>, <tt>'A'</tt>-<tt>'F'</tt>, and
 *           <tt>'a'</tt>-<tt>'f'</tt>)</td></tr>
 *   <tr><th valign=top><i>other</i></th>
 *       <td>The Unicode characters that are not in the US-ASCII character set,
 *           are not control characters (according to the {@link
 *           java.lang.Character#isISOControl(char) Character.isISOControl}
 * 	     method), and are not space characters (according to the {@link
 * 	     java.lang.Character#isSpaceChar(char) Character.isSpaceChar}
 * 	     method)&nbsp;&nbsp;<i>(<b>Deviation from RFC 2396</b>, which is
 * 	     limited to US-ASCII)</i></td></tr>
 * </table></blockquote>
 *
 * <p><a name="legal-chars"></a> The set of all legal URI characters consists of
 * the <i>unreserved</i>, <i>reserved</i>, <i>escaped</i>, and <i>other</i>
 * characters.
 *
 *
 * <h4> Escaped octets, quotation, encoding, and decoding </h4>
 *
 * RFC 2396 allows escaped octets to appear in the user-info, path, query, and
 * fragment components.  Escaping serves two purposes in URIs:
 *
 * <ul>
 *
 *   <li><p> To <i>encode</i> non-US-ASCII characters when a URI is required to
 *   conform strictly to RFC&nbsp;2396 by not containing any <i>other</i>
 *   characters.  </p></li>
 *
 *   <li><p> To <i>quote</i> characters that are otherwise illegal in a
 *   component.  The user-info, path, query, and fragment components differ
 *   slightly in terms of which characters are considered legal and illegal.
 *   </p></li>
 *
 * </ul>
 *
 * These purposes are served in this class by three related operations:
 *
 * <ul>
 *
 *   <li><p><a name="encode"></a> A character is <i>encoded</i> by replacing it
 *   with the sequence of escaped octets that represent that character in the
 *   UTF-8 character set.  The Euro currency symbol (<tt>'&#92;u20AC'</tt>),
 *   for example, is encoded as <tt>"%E2%82%AC"</tt>.  <i>(<b>Deviation from
 *   RFC&nbsp;2396</b>, which does not specify any particular character
 *   set.)</i> </p></li>
 *
 *   <li><p><a name="quote"></a> An illegal character is <i>quoted</i> simply by
 *   encoding it.  The space character, for example, is quoted by replacing it
 *   with <tt>"%20"</tt>.  UTF-8 contains US-ASCII, hence for US-ASCII
 *   characters this transformation has exactly the effect required by
 *   RFC&nbsp;2396. </p></li>
 *
 *   <li><p><a name="decode"></a>
 *   A sequence of escaped octets is <i>decoded</i> by
 *   replacing it with the sequence of characters that it represents in the
 *   UTF-8 character set.  UTF-8 contains US-ASCII, hence decoding has the
 *   effect of de-quoting any quoted US-ASCII characters as well as that of
 *   decoding any encoded non-US-ASCII characters.  If a <a
 *   href="../nio/charset/CharsetDecoder.html#ce">decoding error</a> occurs
 *   when decoding the escaped octets then the erroneous octets are replaced by
 *   <tt>'&#92;uFFFD'</tt>, the Unicode replacement character.  </p></li>
 *
 * </ul>
 *
 * These operations are exposed in the constructors and methods of this class
 * as follows:
 *
 * <ul>
 *
 *   <li><p> The {@link #URI(java.lang.String) <code>single-argument
 *   constructor</code>} requires any illegal characters in its argument to be
 *   quoted and preserves any escaped octets and <i>other</i> characters that
 *   are present.  </p></li>
 *
 *   <li><p> The {@link
 *   #URI(java.lang.String,java.lang.String,java.lang.String,int,java.lang.String,java.lang.String,java.lang.String)
 *   <code>multi-argument constructors</code>} quote illegal characters as
 *   required by the components in which they appear.  The percent character
 *   (<tt>'%'</tt>) is always quoted by these constructors.  Any <i>other</i>
 *   characters are preserved.  </p></li>
 *
 *   <li><p> The {@link #getRawUserInfo() getRawUserInfo}, {@link #getRawPath()
 *   getRawPath}, {@link #getRawQuery() getRawQuery}, {@link #getRawFragment()
 *   getRawFragment}, {@link #getRawAuthority() getRawAuthority}, and {@link
 *   #getRawSchemeSpecificPart() getRawSchemeSpecificPart} methods return the
 *   values of their corresponding components in raw form, without interpreting
 *   any escaped octets.  The strings returned by these methods may contain
 *   both escaped octets and <i>other</i> characters, and will not contain any
 *   illegal characters.  </p></li>
 *
 *   <li><p> The {@link #getUserInfo() getUserInfo}, {@link #getPath()
 *   getPath}, {@link #getQuery() getQuery}, {@link #getFragment()
 *   getFragment}, {@link #getAuthority() getAuthority}, and {@link
 *   #getSchemeSpecificPart() getSchemeSpecificPart} methods decode any escaped
 *   octets in their corresponding components.  The strings returned by these
 *   methods may contain both <i>other</i> characters and illegal characters,
 *   and will not contain any escaped octets.  </p></li>
 *
 *   <li><p> The {@link #toString() toString} method returns a URI string with
 *   all necessary quotation but which may contain <i>other</i> characters.
 *   </p></li>
 *
 *   <li><p> The {@link #toASCIIString() toASCIIString} method returns a fully
 *   quoted and encoded URI string that does not contain any <i>other</i>
 *   characters.  </p></li>
 *
 * </ul>
 *
 *
 * <h4> Identities </h4>
 *
 * For any URI <i>u</i>, it is always the case that
 *
 * <blockquote>
 * <tt>new URI(</tt><i>u</i><tt>.toString()).equals(</tt><i>u</i><tt>)</tt>&nbsp;.
 * </blockquote>
 *
 * For any URI <i>u</i> that does not contain redundant syntax such as two
 * slashes before an empty authority (as in <tt>file:///tmp/</tt>&nbsp;) or a
 * colon following a host name but no port (as in
 * <tt>http://java.sun.com:</tt>&nbsp;), and that does not encode characters
 * except those that must be quoted, the following identities also hold:
 *
 * <blockquote>
 * <tt>new URI(</tt><i>u</i><tt>.getScheme(),<br>
 * &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</tt><i>u</i><tt>.getSchemeSpecificPart(),<br>
 * &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</tt><i>u</i><tt>.getFragment())<br>
 * .equals(</tt><i>u</i><tt>)</tt>
 * </blockquote>
 *
 * in all cases,
 *
 * <blockquote>
 * <tt>new URI(</tt><i>u</i><tt>.getScheme(),<br>
 * &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</tt><i>u</i><tt>.getUserInfo(),&nbsp;</tt><i>u</i><tt>.getAuthority(),<br>
 * &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</tt><i>u</i><tt>.getPath(),&nbsp;</tt><i>u</i><tt>.getQuery(),<br>
 * &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</tt><i>u</i><tt>.getFragment())<br>
 * .equals(</tt><i>u</i><tt>)</tt>
 * </blockquote>
 *
 * if <i>u</i> is hierarchical, and
 *
 * <blockquote>
 * <tt>new URI(</tt><i>u</i><tt>.getScheme(),<br>
 * &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</tt><i>u</i><tt>.getUserInfo(),&nbsp;</tt><i>u</i><tt>.getHost(),&nbsp;</tt><i>u</i><tt>.getPort(),<br>
 * &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</tt><i>u</i><tt>.getPath(),&nbsp;</tt><i>u</i><tt>.getQuery(),<br>
 * &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</tt><i>u</i><tt>.getFragment())<br>
 * .equals(</tt><i>u</i><tt>)</tt>
 * </blockquote>
 *
 * if <i>u</i> is hierarchical and has either no authority or a server-based
 * authority.
 *
 *
 * <h4> URIs, URLs, and URNs </h4>
 *
 * A URI is a uniform resource <i>identifier</i> while a URL is a uniform
 * resource <i>locator</i>.  Hence every URL is a URI, abstractly speaking, but
 * not every URI is a URL.  This is because there is another subcategory of
 * URIs, uniform resource <i>names</i> (URNs), which name resources but do not
 * specify how to locate them.  The <tt>mailto</tt>, <tt>news</tt>, and
 * <tt>isbn</tt> URIs shown above are examples of URNs.
 *
 * <p> The conceptual distinction between URIs and URLs is reflected in the
 * differences between this class and the {@link URL} class.
 *
 * <p> An instance of this class represents a URI reference in the syntactic
 * sense defined by RFC&nbsp;2396.  A URI may be either absolute or relative.
 * A URI string is parsed according to the generic syntax without regard to the
 * scheme, if any, that it specifies.  No lookup of the host, if any, is
 * performed, and no scheme-dependent stream handler is constructed.  Equality,
 * hashing, and comparison are defined strictly in terms of the character
 * content of the instance.  In other words, a URI instance is little more than
 * a structured string that supports the syntactic, scheme-independent
 * operations of comparison, normalization, resolution, and relativization.
 *
 * <p> An instance of the {@link URL} class, by contrast, represents the
 * syntactic components of a URL together with some of the information required
 * to access the resource that it describes.  A URL must be absolute, that is,
 * it must always specify a scheme.  A URL string is parsed according to its
 * scheme.  A stream handler is always established for a URL, and in fact it is
 * impossible to create a URL instance for a scheme for which no handler is
 * available.  Equality and hashing depend upon both the scheme and the
 * Internet address of the host, if any; comparison is not defined.  In other
 * words, a URL is a structured string that supports the syntactic operation of
 * resolution as well as the network I/O operations of looking up the host and
 * opening a connection to the specified resource.
 *
 *
 * @version %I%, %E%
 * @author Mark Reinhold
 * @since 1.4
 *
 * @see <a href="http://ietf.org/rfc/rfc2279.txt"><i>RFC&nbsp;2279: UTF-8, a
 * transformation format of ISO 10646</i></a>, <br><a
 * href="http://www.ietf.org/rfc/rfc2373.txt"><i>RFC&nbsp;2373: IPv6 Addressing
 * Architecture</i></a>, <br><a
 * href="http://www.ietf.org/rfc/rfc2396.txt""><i>RFC&nbsp;2396: Uniform
 * Resource Identifiers (URI): Generic Syntax</i></a>, <br><a
 * href="http://www.ietf.org/rfc/rfc2732.txt"><i>RFC&nbsp;2732: Format for
 * Literal IPv6 Addresses in URLs</i></a>, <br><a
 * href="URISyntaxException.html">URISyntaxException</a>
 */

public final class URI
    implements Comparable<URI>, Serializable
{

    // Note: Comments containing the word "ASSERT" indicate places where a
    // throw of an InternalError should be replaced by an appropriate assertion
    // statement once asserts are enabled in the build.

    static final long serialVersionUID = -6052424284110960213L;


    // -- Properties and components of this instance --

    // Components of all URIs: [<scheme>:]<scheme-specific-part>[#<fragment>]
    private transient String scheme;		// null ==> relative URI
    private transient String fragment;

    // Hierarchical URI components: [//<authority>]<path>[?<query>]
    private transient String authority;		// Registry or server

    // Server-based authority: [<userInfo>@]<host>[:<port>]
    private transient String userInfo;
    private transient String host;		// null ==> registry-based
    private transient int port = -1;		// -1 ==> undefined

    // Remaining components of hierarchical URIs
    private transient String path;		// null ==> opaque
    private transient String query;

    // The remaining fields may be computed on demand

    private volatile transient String schemeSpecificPart;
    private volatile transient int hash;	// Zero ==> undefined

    private volatile transient String decodedUserInfo = null;
    private volatile transient String decodedAuthority = null;
    private volatile transient String decodedPath = null;
    private volatile transient String decodedQuery = null;
    private volatile transient String decodedFragment = null;
    private volatile transient String decodedSchemeSpecificPart = null;

    /**
     * The string form of this URI.
     *
     * @serial
     */
    private volatile String string;		// The only serializable field



    // -- Constructors and factories --

    private URI() { }				// Used internally

    /**
     * Constructs a URI by parsing the given string.
     *
     * <p> This constructor parses the given string exactly as specified by the
     * grammar in <a
     * href="http://www.ietf.org/rfc/rfc2396.txt">RFC&nbsp;2396</a>,
     * Appendix&nbsp;A, <b><i>except for the following deviations:</i></b> </p>
     *
     * <ul type=disc>
     *
     *   <li><p> An empty authority component is permitted as long as it is
     *   followed by a non-empty path, a query component, or a fragment
     *   component.  This allows the parsing of URIs such as
     *   <tt>"file:///foo/bar"</tt>, which seems to be the intent of
     *   RFC&nbsp;2396 although the grammar does not permit it.  If the
     *   authority component is empty then the user-information, host, and port
     *   components are undefined. </p></li>
     *
     *   <li><p> Empty relative paths are permitted; this seems to be the
     *   intent of RFC&nbsp;2396 although the grammar does not permit it.  The
     *   primary consequence of this deviation is that a standalone fragment
     *   such as <tt>"#foo"</tt> parses as a relative URI with an empty path
     *   and the given fragment, and can be usefully <a
     *   href="#resolve-frag">resolved</a> against a base URI.
     *
     *   <li><p> IPv4 addresses in host components are parsed rigorously, as
     *   specified by <a
     *   href="http://www.ietf.org/rfc/rfc2732.txt">RFC&nbsp;2732</a>: Each
     *   element of a dotted-quad address must contain no more than three
     *   decimal digits.  Each element is further constrained to have a value
     *   no greater than 255. </p></li>
     *
     *   <li> <p> Hostnames in host components that comprise only a single
     *   domain label are permitted to start with an <i>alphanum</i> 
     *   character. This seems to be the intent of <a
     *   href="http://www.ietf.org/rfc/rfc2396.txt">RFC&nbsp;2396</a>
     *   section&nbsp;3.2.2 although the grammar does not permit it. The
     *   consequence of this deviation is that the authority component of a
     *   hierarchical URI such as <tt>s://123</tt>, will parse as a server-based 
     *   authority. </p></li>
     *
     *   <li><p> IPv6 addresses are permitted for the host component.  An IPv6
     *   address must be enclosed in square brackets (<tt>'['</tt> and
     *   <tt>']'</tt>) as specified by <a
     *   href="http://www.ietf.org/rfc/rfc2732.txt">RFC&nbsp;2732</a>.  The
     *   IPv6 address itself must parse according to <a
     *   href="http://www.ietf.org/rfc/rfc2373.txt">RFC&nbsp;2373</a>.  IPv6
     *   addresses are further constrained to describe no more than sixteen
     *   bytes of address information, a constraint implicit in RFC&nbsp;2373
     *   but not expressible in the grammar. </p></li>
     *
     *   <li><p> Characters in the <i>other</i> category are permitted wherever
     *   RFC&nbsp;2396 permits <i>escaped</i> octets, that is, in the
     *   user-information, path, query, and fragment components, as well as in
     *   the authority component if the authority is registry-based.  This
     *   allows URIs to contain Unicode characters beyond those in the US-ASCII
     *   character set. </p></li>
     *
     * </ul>
     *
     * @param  str   The string to be parsed into a URI
     *
     * @throws  NullPointerException
     *          If <tt>str</tt> is <tt>null</tt>
     *
     * @throws  URISyntaxException
     *          If the given string violates RFC&nbsp;2396, as augmented
     *          by the above deviations
     */
    public URI(String str) throws URISyntaxException {
	new Parser(str).parse(false);
    }

    /**
     * Constructs a hierarchical URI from the given components.
     *
     * <p> If a scheme is given then the path, if also given, must either be
     * empty or begin with a slash character (<tt>'/'</tt>).  Otherwise a
     * component of the new URI may be left undefined by passing <tt>null</tt>
     * for the corresponding parameter or, in the case of the <tt>port</tt>
     * parameter, by passing <tt>-1</tt>.
     *
     * <p> This constructor first builds a URI string from the given components
     * according to the rules specified in <a
     * href="http://www.ietf.org/rfc/rfc2396.txt">RFC&nbsp;2396</a>,
     * section&nbsp;5.2, step&nbsp;7: </p>
     *
     * <ol>
     *
     *   <li><p> Initially, the result string is empty. </p></li>
     *
     *   <li><p> If a scheme is given then it is appended to the result,
     *   followed by a colon character (<tt>':'</tt>).  </p></li>
     *
     *   <li><p> If user information, a host, or a port are given then the
     *   string <tt>"//"</tt> is appended.  </p></li>
     *
     *   <li><p> If user information is given then it is appended, followed by
     *   a commercial-at character (<tt>'@'</tt>).  Any character not in the
     *   <i>unreserved</i>, <i>punct</i>, <i>escaped</i>, or <i>other</i>
     *   categories is <a href="#quote">quoted</a>.  </p></li>
     *
     *   <li><p> If a host is given then it is appended.  If the host is a
     *   literal IPv6 address but is not enclosed in square brackets
     *   (<tt>'['</tt> and <tt>']'</tt>) then the square brackets are added.
     *   </p></li>
     *
     *   <li><p> If a port number is given then a colon character
     *   (<tt>':'</tt>) is appended, followed by the port number in decimal.
     *   </p></li>
     *
     *   <li><p> If a path is given then it is appended.  Any character not in
     *   the <i>unreserved</i>, <i>punct</i>, <i>escaped</i>, or <i>other</i>
     *   categories, and not equal to the slash character (<tt>'/'</tt>) or the
     *   commercial-at character (<tt>'@'</tt>), is quoted.  </p></li>
     *
     *   <li><p> If a query is given then a question-mark character
     *   (<tt>'?'</tt>) is appended, followed by the query.  Any character that
     *   is not a <a href="#legal-chars">legal URI character</a> is quoted.
     *   </p></li>
     *
     *   <li><p> Finally, if a fragment is given then a hash character
     *   (<tt>'#'</tt>) is appended, followed by the fragment.  Any character
     *   that is not a legal URI character is quoted.  </p></li>
     *
     * </ol>
     *
     * <p> The resulting URI string is then parsed as if by invoking the {@link
     * #URI(String)} constructor and then invoking the {@link
     * #parseServerAuthority()} method upon the result; this may cause a {@link
     * URISyntaxException} to be thrown.  </p>
     *
     * @param   scheme    Scheme name
     * @param   userInfo  User name and authorization information
     * @param   host      Host name
     * @param   port      Port number
     * @param   path      Path
     * @param   query     Query
     * @param   fragment  Fragment
     *
     * @throws URISyntaxException
     *         If both a scheme and a path are given but the path is relative,
     *         if the URI string constructed from the given components violates
     *         RFC&nbsp;2396, or if the authority component of the string is
     *         present but cannot be parsed as a server-based authority
     */
    public URI(String scheme,
               String userInfo, String host, int port,
               String path, String query, String fragment)
	throws URISyntaxException
    {
	String s = toString(scheme, null,
			    null, userInfo, host, port,
			    path, query, fragment);
	checkPath(s, scheme, path);
	new Parser(s).parse(true);
    }

    /**
     * Constructs a hierarchical URI from the given components.
     *
     * <p> If a scheme is given then the path, if also given, must either be
     * empty or begin with a slash character (<tt>'/'</tt>).  Otherwise a
     * component of the new URI may be left undefined by passing <tt>null</tt>
     * for the corresponding parameter.
     *
     * <p> This constructor first builds a URI string from the given components
     * according to the rules specified in <a
     * href="http://www.ietf.org/rfc/rfc2396.txt">RFC&nbsp;2396</a>,
     * section&nbsp;5.2, step&nbsp;7: </p>
     *
     * <ol>
     *
     *   <li><p> Initially, the result string is empty.  </p></li>
     *
     *   <li><p> If a scheme is given then it is appended to the result,
     *   followed by a colon character (<tt>':'</tt>).  </p></li>
     *
     *   <li><p> If an authority is given then the string <tt>"//"</tt> is
     *   appended, followed by the authority.  If the authority contains a
     *   literal IPv6 address then the address must be enclosed in square
     *   brackets (<tt>'['</tt> and <tt>']'</tt>).  Any character not in the
     *   <i>unreserved</i>, <i>punct</i>, <i>escaped</i>, or <i>other</i>
     *   categories, and not equal to the commercial-at character
     *   (<tt>'@'</tt>), is <a href="#quote">quoted</a>.  </p></li>
     *
     *   <li><p> If a path is given then it is appended.  Any character not in
     *   the <i>unreserved</i>, <i>punct</i>, <i>escaped</i>, or <i>other</i>
     *   categories, and not equal to the slash character (<tt>'/'</tt>) or the
     *   commercial-at character (<tt>'@'</tt>), is quoted.  </p></li>
     *
     *   <li><p> If a query is given then a question-mark character
     *   (<tt>'?'</tt>) is appended, followed by the query.  Any character that
     *   is not a <a href="#legal-chars">legal URI character</a> is quoted.
     *   </p></li>
     *
     *   <li><p> Finally, if a fragment is given then a hash character
     *   (<tt>'#'</tt>) is appended, followed by the fragment.  Any character
     *   that is not a legal URI character is quoted.  </p></li>
     *
     * </ol>
     *
     * <p> The resulting URI string is then parsed as if by invoking the {@link
     * #URI(String)} constructor and then invoking the {@link
     * #parseServerAuthority()} method upon the result; this may cause a {@link
     * URISyntaxException} to be thrown.  </p>
     *
     * @param   scheme     Scheme name
     * @param   authority  Authority
     * @param   path       Path
     * @param   query      Query
     * @param   fragment   Fragment
     *
     * @throws URISyntaxException
     *         If both a scheme and a path are given but the path is relative,
     *         if the URI string constructed from the given components violates
     *         RFC&nbsp;2396, or if the authority component of the string is
     *         present but cannot be parsed as a server-based authority
     */
    public URI(String scheme,
	       String authority,
	       String path, String query, String fragment)
	throws URISyntaxException
    {
	String s = toString(scheme, null,
			    authority, null, null, -1,
			    path, query, fragment);
	checkPath(s, scheme, path);
	new Parser(s).parse(false);
    }

    /**
     * Constructs a hierarchical URI from the given components.
     *
     * <p> A component may be left undefined by passing <tt>null</tt>.
     *
     * <p> This convenience constructor works as if by invoking the
     * seven-argument constructor as follows:
     *
     * <blockquote><tt>
     * new&nbsp;{@link #URI(String, String, String, int, String, String, String)
     * URI}(scheme,&nbsp;null,&nbsp;host,&nbsp;-1,&nbsp;path,&nbsp;null,&nbsp;fragment);
     * </tt></blockquote>
     *
     * @param   scheme    Scheme name
     * @param   host      Host name
     * @param   path      Path
     * @param   fragment  Fragment
     *
     * @throws  URISyntaxException
     *          If the URI string constructed from the given components
     *          violates RFC&nbsp;2396
     */
    public URI(String scheme, String host, String path, String fragment)
	throws URISyntaxException
    {
	this(scheme, null, host, -1, path, null, fragment);
    }

    /**
     * Constructs a URI from the given components.
     *
     * <p> A component may be left undefined by passing <tt>null</tt>.
     *
     * <p> This constructor first builds a URI in string form using the given
     * components as follows:  </p>
     *
     * <ol>
     *
     *   <li><p> Initially, the result string is empty.  </p></li>
     *
     *   <li><p> If a scheme is given then it is appended to the result,
     *   followed by a colon character (<tt>':'</tt>).  </p></li>
     *
     *   <li><p> If a scheme-specific part is given then it is appended.  Any
     *   character that is not a <a href="#legal-chars">legal URI character</a>
     *   is <a href="#quote">quoted</a>.  </p></li>
     *
     *   <li><p> Finally, if a fragment is given then a hash character
     *   (<tt>'#'</tt>) is appended to the string, followed by the fragment.
     *   Any character that is not a legal URI character is quoted.  </p></li>
     *
     * </ol>
     *
     * <p> The resulting URI string is then parsed in order to create the new
     * URI instance as if by invoking the {@link #URI(String)} constructor;
     * this may cause a {@link URISyntaxException} to be thrown.  </p>
     *
     * @param   scheme    Scheme name
     * @param   ssp       Scheme-specific part
     * @param   fragment  Fragment
     *
     * @throws  URISyntaxException
     *          If the URI string constructed from the given components
     *          violates RFC&nbsp;2396
     */
    public URI(String scheme, String ssp, String fragment)
	throws URISyntaxException
    {
	new Parser(toString(scheme, ssp,
			    null, null, null, -1,
			    null, null, fragment))
	    .parse(false);
    }

    /**
     * Creates a URI by parsing the given string.
     *
     * <p> This convenience factory method works as if by invoking the {@link
     * #URI(String)} constructor; any {@link URISyntaxException} thrown by the
     * constructor is caught and wrapped in a new {@link
     * IllegalArgumentException} object, which is then thrown.
     *
     * <p> This method is provided for use in situations where it is known that
     * the given string is a legal URI, for example for URI constants declared
     * within in a program, and so it would be considered a programming error
     * for the string not to parse as such.  The constructors, which throw
     * {@link URISyntaxException} directly, should be used situations where a
     * URI is being constructed from user input or from some other source that
     * may be prone to errors.  </p>
     *
     * @param  str   The string to be parsed into a URI
     * @return The new URI
     *
     * @throws  NullPointerException
     *          If <tt>str</tt> is <tt>null</tt>
     *
     * @throws  IllegalArgumentException
     *          If the given string violates RFC&nbsp;2396
     */
    public static URI create(String str) {
	try {
	    return new URI(str);
	} catch (URISyntaxException x) {
	    IllegalArgumentException y = new IllegalArgumentException();
	    y.initCause(x);
	    throw y;
	}
    }


    // -- Operations --

    /**
     * Attempts to parse this URI's authority component, if defined, into
     * user-information, host, and port components.
     *
     * <p> If this URI's authority component has already been recognized as
     * being server-based then it will already have been parsed into
     * user-information, host, and port components.  In this case, or if this
     * URI has no authority component, this method simply returns this URI.
     *
     * <p> Otherwise this method attempts once more to parse the authority
     * component into user-information, host, and port components, and throws
     * an exception describing why the authority component could not be parsed
     * in that way.
     *
     * <p> This method is provided because the generic URI syntax specified in
     * <a href="http://www.ietf.org/rfc/rfc2396.txt">RFC&nbsp;2396</a>
     * cannot always distinguish a malformed server-based authority from a
     * legitimate registry-based authority.  It must therefore treat some
     * instances of the former as instances of the latter.  The authority
     * component in the URI string <tt>"//foo:bar"</tt>, for example, is not a
     * legal server-based authority but it is legal as a registry-based
     * authority.
     *
     * <p> In many common situations, for example when working URIs that are
     * known to be either URNs or URLs, the hierarchical URIs being used will
     * always be server-based.  They therefore must either be parsed as such or
     * treated as an error.  In these cases a statement such as
     *
     * <blockquote>
     * <tt>URI </tt><i>u</i><tt> = new URI(str).parseServerAuthority();</tt>
     * </blockquote>
     *
     * <p> can be used to ensure that <i>u</i> always refers to a URI that, if
     * it has an authority component, has a server-based authority with proper
     * user-information, host, and port components.  Invoking this method also
     * ensures that if the authority could not be parsed in that way then an
     * appropriate diagnostic message can be issued based upon the exception
     * that is thrown. </p>
     *
     * @return  A URI whose authority field has been parsed
     *          as a server-based authority
     *
     * @throws  URISyntaxException
     *          If the authority component of this URI is defined
     *          but cannot be parsed as a server-based authority
     *          according to RFC&nbsp;2396
     */
    public URI parseServerAuthority()
	throws URISyntaxException
    {
	// We could be clever and cache the error message and index from the
	// exception thrown during the original parse, but that would require
	// either more fields or a more-obscure representation.
	if ((host != null) || (authority == null))
	    return this;
	defineString();
	new Parser(string).parse(true);
	return this;
    }

    /**
     * Normalizes this URI's path.
     *
     * <p> If this URI is opaque, or if its path is already in normal form,
     * then this URI is returned.  Otherwise a new URI is constructed that is
     * identical to this URI except that its path is computed by normalizing
     * this URI's path in a manner consistent with <a
     * href="http://www.ietf.org/rfc/rfc2396.txt">RFC&nbsp;2396</a>,
     * section&nbsp;5.2, step&nbsp;6, sub-steps&nbsp;c through&nbsp;f; that is:
     * </p>
     *
     * <ol>
     *
     *   <li><p> All <tt>"."</tt> segments are removed. </p></li>
     *
     *   <li><p> If a <tt>".."</tt> segment is preceded by a non-<tt>".."</tt>
     *   segment then both of these segments are removed.  This step is
     *   repeated until it is no longer applicable. </p></li>
     *
     *   <li><p> If the path is relative, and if its first segment contains a
     *   colon character (<tt>':'</tt>), then a <tt>"."</tt> segment is
     *   prepended.  This prevents a relative URI with a path such as
     *   <tt>"a:b/c/d"</tt> from later being re-parsed as an opaque URI with a
     *   scheme of <tt>"a"</tt> and a scheme-specific part of <tt>"b/c/d"</tt>.
     *   <b><i>(Deviation from RFC&nbsp;2396)</i></b> </p></li>
     *
     * </ol>
     *
     * <p> A normalized path will begin with one or more <tt>".."</tt> segments
     * if there were insufficient non-<tt>".."</tt> segments preceding them to
     * allow their removal.  A normalized path will begin with a <tt>"."</tt>
     * segment if one was inserted by step 3 above.  Otherwise, a normalized
     * path will not contain any <tt>"."</tt> or <tt>".."</tt> segments. </p>
     *
     * @return  A URI equivalent to this URI,
     *          but whose path is in normal form
     */
    public URI normalize() {
	return normalize(this);
    }

    /**
     * Resolves the given URI against this URI.
     *
     * <p> If the given URI is already absolute, or if this URI is opaque, then
     * the given URI is returned.
     *
     * <p><a name="resolve-frag"></a> If the given URI's fragment component is
     * defined, its path component is empty, and its scheme, authority, and
     * query components are undefined, then a URI with the given fragment but
     * with all other components equal to those of this URI is returned.  This
     * allows a URI representing a standalone fragment reference, such as
     * <tt>"#foo"</tt>, to be usefully resolved against a base URI.
     *
     * <p> Otherwise this method constructs a new hierarchical URI in a manner
     * consistent with <a
     * href="http://www.ietf.org/rfc/rfc2396.txt">RFC&nbsp;2396</a>,
     * section&nbsp;5.2; that is: </p>
     *
     * <ol>
     *
     *   <li><p> A new URI is constructed with this URI's scheme and the given
     *   URI's query and fragment components. </p></li>
     *
     *   <li><p> If the given URI has an authority component then the new URI's
     *   authority and path are taken from the given URI. </p></li>
     *
     *   <li><p> Otherwise the new URI's authority component is copied from
     *   this URI, and its path is computed as follows: </p></li>
     *
     *   <ol type=a>
     *
     *     <li><p> If the given URI's path is absolute then the new URI's path
     *     is taken from the given URI. </p></li>
     *
     *     <li><p> Otherwise the given URI's path is relative, and so the new
     *     URI's path is computed by resolving the path of the given URI
     *     against the path of this URI.  This is done by concatenating all but
     *     the last segment of this URI's path, if any, with the given URI's
     *     path and then normalizing the result as if by invoking the {@link
     *     #normalize() normalize} method. </p></li>
     *
     *   </ol>
     *
     * </ol>
     *
     * <p> The result of this method is absolute if, and only if, either this
     * URI is absolute or the given URI is absolute.  </p>
     *
     * @param  uri  The URI to be resolved against this URI
     * @return The resulting URI
     *
     * @throws  NullPointerException
     *          If <tt>uri</tt> is <tt>null</tt>
     */
    public URI resolve(URI uri) {
	return resolve(this, uri);
    }

    /**
     * Constructs a new URI by parsing the given string and then resolving it
     * against this URI.
     *
     * <p> This convenience method works as if invoking it were equivalent to
     * evaluating the expression <tt>{@link #resolve(java.net.URI)
     * resolve}(URI.{@link #create(String) create}(str))</tt>. </p>
     *
     * @param  str   The string to be parsed into a URI
     * @return The resulting URI
     *
     * @throws  NullPointerException
     *          If <tt>str</tt> is <tt>null</tt>
     *
     * @throws  IllegalArgumentException
     *          If the given string violates RFC&nbsp;2396
     */
    public URI resolve(String str) {
	return resolve(URI.create(str));
    }

    /**
     * Relativizes the given URI against this URI.
     *
     * <p> The relativization of the given URI against this URI is computed as
     * follows: </p>
     *
     * <ol>
     *
     *   <li><p> If either this URI or the given URI are opaque, or if the
     *   scheme and authority components of the two URIs are not identical, or
     *   if the path of this URI is not a prefix of the path of the given URI,
     *   then the given URI is returned. </p></li>
     *
     *   <li><p> Otherwise a new relative hierarchical URI is constructed with
     *   query and fragment components taken from the given URI and with a path
     *   component computed by removing this URI's path from the beginning of
     *   the given URI's path. </p></li>
     *
     * </ol>
     *
     * @param  uri  The URI to be relativized against this URI
     * @return The resulting URI
     *
     * @throws  NullPointerException
     *          If <tt>uri</tt> is <tt>null</tt>
     */
    public URI relativize(URI uri) {
	return relativize(this, uri);
    }

    /**
     * Constructs a URL from this URI.
     *
     * <p> This convenience method works as if invoking it were equivalent to
     * evaluating the expression <tt>new&nbsp;URL(this.toString())</tt> after
     * first checking that this URI is absolute. </p>
     *
     * @return  A URL constructed from this URI
     *
     * @throws  IllegalArgumentException
     *          If this URL is not absolute
     *
     * @throws  MalformedURLException
     *          If a protocol handler for the URL could not be found,
     *          or if some other error occurred while constructing the URL
     */
    public URL toURL()
	throws MalformedURLException {
	if (!isAbsolute())
	    throw new IllegalArgumentException("URI is not absolute");
	return new URL(toString());
    }

    // -- Component access methods --

    /**
     * Returns the scheme component of this URI.
     *
     * <p> The scheme component of a URI, if defined, only contains characters
     * in the <i>alphanum</i> category and in the string <tt>"-.+"</tt>.  A
     * scheme always starts with an <i>alpha</i> character. <p>
     *
     * The scheme component of a URI cannot contain escaped octets, hence this
     * method does not perform any decoding.
     *
     * @return  The scheme component of this URI,
     *          or <tt>null</tt> if the scheme is undefined
     */
    public String getScheme() {
	return scheme;
    }

    /**
     * Tells whether or not this URI is absolute.
     *
     * <p> A URI is absolute if, and only if, it has a scheme component. </p>
     *
     * @return  <tt>true</tt> if, and only if, this URI is absolute
     */
    public boolean isAbsolute() {
	return scheme != null;
    }

    /**
     * Tells whether or not this URI is opaque.
     *
     * <p> A URI is opaque if, and only if, it is absolute and its
     * scheme-specific part does not begin with a slash character ('/').
     * An opaque URI has a scheme, a scheme-specific part, and possibly
     * a fragment; all other components are undefined. </p>
     *
     * @return  <tt>true</tt> if, and only if, this URI is opaque
     */
    public boolean isOpaque() {
        return path == null;
    }

    /**
     * Returns the raw scheme-specific part of this URI.  The scheme-specific
     * part is never undefined, though it may be empty.
     *
     * <p> The scheme-specific part of a URI only contains legal URI
     * characters. </p>
     *
     * @return  The raw scheme-specific part of this URI
     *          (never <tt>null</tt>)
     */
    public String getRawSchemeSpecificPart() {
	defineSchemeSpecificPart();
	return schemeSpecificPart;
    }

    /**
     * Returns the decoded scheme-specific part of this URI.
     *
     * <p> The string returned by this method is equal to that returned by the
     * {@link #getRawSchemeSpecificPart() getRawSchemeSpecificPart} method
     * except that all sequences of escaped octets are <a
     * href="#decode">decoded</a>.  </p>
     *
     * @return  The decoded scheme-specific part of this URI
     *          (never <tt>null</tt>)
     */
    public String getSchemeSpecificPart() {
	if (decodedSchemeSpecificPart == null)
	    decodedSchemeSpecificPart = decode(getRawSchemeSpecificPart());
	return decodedSchemeSpecificPart;
    }

    /**
     * Returns the raw authority component of this URI.
     *
     * <p> The authority component of a URI, if defined, only contains the
     * commercial-at character (<tt>'@'</tt>) and characters in the
     * <i>unreserved</i>, <i>punct</i>, <i>escaped</i>, and <i>other</i>
     * categories.  If the authority is server-based then it is further
     * constrained to have valid user-information, host, and port
     * components. </p>
     *
     * @return  The raw authority component of this URI,
     *          or <tt>null</tt> if the authority is undefined
     */
    public String getRawAuthority() {
	return authority;
    }

    /**
     * Returns the decoded authority component of this URI.
     *
     * <p> The string returned by this method is equal to that returned by the
     * {@link #getRawAuthority() getRawAuthority} method except that all
     * sequences of escaped octets are <a href="#decode">decoded</a>.  </p>
     *
     * @return  The decoded authority component of this URI,
     *          or <tt>null</tt> if the authority is undefined
     */
    public String getAuthority() {
	if (decodedAuthority == null)
	    decodedAuthority = decode(authority);
	return decodedAuthority;
    }

    /**
     * Returns the raw user-information component of this URI.
     *
     * <p> The user-information component of a URI, if defined, only contains
     * characters in the <i>unreserved</i>, <i>punct</i>, <i>escaped</i>, and
     * <i>other</i> categories. </p>
     *
     * @return  The raw user-information component of this URI,
     *          or <tt>null</tt> if the user information is undefined
     */
    public String getRawUserInfo() {
	return userInfo;
    }

    /**
     * Returns the decoded user-information component of this URI.
     *
     * <p> The string returned by this method is equal to that returned by the
     * {@link #getRawUserInfo() getRawUserInfo} method except that all
     * sequences of escaped octets are <a href="#decode">decoded</a>.  </p>
     *
     * @return  The decoded user-information component of this URI,
     *          or <tt>null</tt> if the user information is undefined
     */
    public String getUserInfo() {
	if ((decodedUserInfo == null) && (userInfo != null))
	    decodedUserInfo = decode(userInfo);
	return decodedUserInfo;
    }

    /**
     * Returns the host component of this URI.
     *
     * <p> The host component of a URI, if defined, will have one of the
     * following forms: </p>
     *
     * <ul type=disc>
     *
     *   <li><p> A domain name consisting of one or more <i>labels</i>
     *   separated by period characters (<tt>'.'</tt>), optionally followed by
     *   a period character.  Each label consists of <i>alphanum</i> characters
     *   as well as hyphen characters (<tt>'-'</tt>), though hyphens never
     *   occur as the first or last characters in a label. The rightmost
     *   label of a domain name consisting of two or more labels, begins
     *   with an <i>alpha</i> character. </li>
     *
     *   <li><p> A dotted-quad IPv4 address of the form
     *   <i>digit</i><tt>+.</tt><i>digit</i><tt>+.</tt><i>digit</i><tt>+.</tt><i>digit</i><tt>+</tt>,
     *   where no <i>digit</i> sequence is longer than three characters and no
     *   sequence has a value larger than 255. </p></li>
     *
     *   <li><p> An IPv6 address enclosed in square brackets (<tt>'['</tt> and
     *   <tt>']'</tt>) and consisting of hexadecimal digits, colon characters
     *   (<tt>':'</tt>), and possibly an embedded IPv4 address.  The full
     *   syntax of IPv6 addresses is specified in <a
     *   href="http://www.ietf.org/rfc/rfc2373.txt"><i>RFC&nbsp;2373: IPv6
     *   Addressing Architecture</i></a>.  </p></li>
     *
     * </ul>
     *
     * The host component of a URI cannot contain escaped octets, hence this
     * method does not perform any decoding.
     *
     * @return  The host component of this URI,
     *          or <tt>null</tt> if the host is undefined
     */
    public String getHost() {
	return host;
    }

    /**
     * Returns the port number of this URI.
     *
     * <p> The port component of a URI, if defined, is a non-negative
     * integer. </p>
     *
     * @return  The port component of this URI,
     *          or <tt>-1</tt> if the port is undefined
     */
    public int getPort() {
	return port;
    }

    /**
     * Returns the raw path component of this URI.
     *
     * <p> The path component of a URI, if defined, only contains the slash
     * character (<tt>'/'</tt>), the commercial-at character (<tt>'@'</tt>),
     * and characters in the <i>unreserved</i>, <i>punct</i>, <i>escaped</i>,
     * and <i>other</i> categories. </p>
     *
     * @return  The path component of this URI,
     *          or <tt>null</tt> if the path is undefined
     */
    public String getRawPath() {
	return path;
    }

    /**
     * Returns the decoded path component of this URI.
     *
     * <p> The string returned by this method is equal to that returned by the
     * {@link #getRawPath() getRawPath} method except that all sequences of
     * escaped octets are <a href="#decode">decoded</a>.  </p>
     *
     * @return  The decoded path component of this URI,
     *          or <tt>null</tt> if the path is undefined
     */
    public String getPath() {
	if ((decodedPath == null) && (path != null))
	    decodedPath = decode(path);
	return decodedPath;
    }

    /**
     * Returns the raw query component of this URI.
     *
     * <p> The query component of a URI, if defined, only contains legal URI
     * characters. </p>
     *
     * @return  The raw query component of this URI,
     *          or <tt>null</tt> if the query is undefined
     */
    public String getRawQuery() {
	return query;
    }

    /**
     * Returns the decoded query component of this URI.
     *
     * <p> The string returned by this method is equal to that returned by the
     * {@link #getRawQuery() getRawQuery} method except that all sequences of
     * escaped octets are <a href="#decode">decoded</a>.  </p>
     *
     * @return  The decoded query component of this URI,
     *          or <tt>null</tt> if the query is undefined
     */
    public String getQuery() {
	if ((decodedQuery == null) && (query != null))
	    decodedQuery = decode(query);
	return decodedQuery;
    }

    /**
     * Returns the raw fragment component of this URI.
     *
     * <p> The fragment component of a URI, if defined, only contains legal URI
     * characters. </p>
     *
     * @return  The raw fragment component of this URI,
     *          or <tt>null</tt> if the fragment is undefined
     */
    public String getRawFragment() {
	return fragment;
    }

    /**
     * Returns the decoded fragment component of this URI.
     *
     * <p> The string returned by this method is equal to that returned by the
     * {@link #getRawFragment() getRawFragment} method except that all
     * sequences of escaped octets are <a href="#decode">decoded</a>.  </p>
     *
     * @return  The decoded fragment component of this URI,
     *          or <tt>null</tt> if the fragment is undefined
     */
    public String getFragment() {
	if ((decodedFragment == null) && (fragment != null))
	    decodedFragment = decode(fragment);
	return decodedFragment;
    }


    // -- Equality, comparison, hash code, toString, and serialization --

    /**
     * Tests this URI for equality with another object.
     *
     * <p> If the given object is not a URI then this method immediately
     * returns <tt>false</tt>.
     *
     * <p> For two URIs to be considered equal requires that either both are
     * opaque or both are hierarchical.  Their schemes must either both be
     * undefined or else be equal without regard to case. Their fragments
     * must either both be undefined or else be equal.
     *
     * <p> For two opaque URIs to be considered equal, their scheme-specific
     * parts must be equal.
     *
     * <p> For two hierarchical URIs to be considered equal, their paths must
     * be equal and their queries must either both be undefined or else be
     * equal.  Their authorities must either both be undefined, or both be
     * registry-based, or both be server-based.  If their authorities are
     * defined and are registry-based, then they must be equal.  If their
     * authorities are defined and are server-based, then their hosts must be
     * equal without regard to case, their port numbers must be equal, and
     * their user-information components must be equal.
     *
     * <p> When testing the user-information, path, query, fragment, authority,
     * or scheme-specific parts of two URIs for equality, the raw forms rather
     * than the encoded forms of these components are compared and the
     * hexadecimal digits of escaped octets are compared without regard to
     * case.
     *
     * <p> This method satisfies the general contract of the {@link
     * java.lang.Object#equals(Object) Object.equals} method. </p>
     *
     * @param   ob   The object to which this object is to be compared
     *
     * @return  <tt>true</tt> if, and only if, the given object is a URI that
     *          is identical to this URI
     */
    public boolean equals(Object ob) {
	if (ob == this)
	    return true;
	if (!(ob instanceof URI))
	    return false;
	URI that = (URI)ob;
	if (this.isOpaque() != that.isOpaque()) return false;
	if (!equalIgnoringCase(this.scheme, that.scheme)) return false;
	if (!equal(this.fragment, that.fragment)) return false;

	// Opaque
	if (this.isOpaque())
	    return equal(this.schemeSpecificPart, that.schemeSpecificPart);

	// Hierarchical
	if (!equal(this.path, that.path)) return false;
	if (!equal(this.query, that.query)) return false;

	// Authorities
	if (this.authority == that.authority) return true;
	if (this.host != null) {
	    // Server-based
	    if (!equal(this.userInfo, that.userInfo)) return false;
	    if (!equalIgnoringCase(this.host, that.host)) return false;
	    if (this.port != that.port) return false;
	} else if (this.authority != null) {
	    // Registry-based
	    if (!equal(this.authority, that.authority)) return false;
	} else if (this.authority != that.authority) {
	    return false;
	}

	return true;
    }

    /**
     * Returns a hash-code value for this URI.  The hash code is based upon all
     * of the URI's components, and satisfies the general contract of the
     * {@link java.lang.Object#hashCode() Object.hashCode} method.
     *
     * @return  A hash-code value for this URI
     */
    public int hashCode() {
	if (hash != 0)
	    return hash;
	int h = hashIgnoringCase(0, scheme);
	h = hash(h, fragment);
	if (isOpaque()) {
	    h = hash(h, schemeSpecificPart);
	} else {
	    h = hash(h, path);
	    h = hash(h, query);
	    if (host != null) {
		h = hash(h, userInfo);
		h = hashIgnoringCase(h, host);
		h += 1949 * port;
	    } else {
		h = hash(h, authority);
	    }
	}
	hash = h;
	return h;
    }

    /**
     * Compares this URI to another object, which must be a URI.
     *
     * <p> When comparing corresponding components of two URIs, if one
     * component is undefined but the other is defined then the first is
     * considered to be less than the second.  Unless otherwise noted, string
     * components are ordered according to their natural, case-sensitive
     * ordering as defined by the {@link java.lang.String#compareTo(Object)
     * String.compareTo} method.  String components that are subject to
     * encoding are compared by comparing their raw forms rather than their
     * encoded forms.
     *
     * <p> The ordering of URIs is defined as follows: </p>
     *
     * <ul type=disc>
     *
     *   <li><p> Two URIs with different schemes are ordered according the
     *   ordering of their schemes, without regard to case. </p></li>
     *
     *   <li><p> A hierarchical URI is considered to be less than an opaque URI
     *   with an identical scheme. </p></li>
     *
     *   <li><p> Two opaque URIs with identical schemes are ordered according
     *   to the ordering of their scheme-specific parts. </p></li>
     *
     *   <li><p> Two opaque URIs with identical schemes and scheme-specific
     *   parts are ordered according to the ordering of their
     *   fragments. </p></li>
     *
     *   <li><p> Two hierarchical URIs with identical schemes are ordered
     *   according to the ordering of their authority components: </p></li>
     *
     *   <ul type=disc>
     *
     *     <li><p> If both authority components are server-based then the URIs
     *     are ordered according to their user-information components; if these
     *     components are identical then the URIs are ordered according to the
     *     ordering of their hosts, without regard to case; if the hosts are
     *     identical then the URIs are ordered according to the ordering of
     *     their ports. </p></li>
     *
     *     <li><p> If one or both authority components are registry-based then
     *     the URIs are ordered according to the ordering of their authority
     *     components. </p></li>
     *
     *   </ul>
     *
     *   <li><p> Finally, two hierarchical URIs with identical schemes and
     *   authority components are ordered according to the ordering of their
     *   paths; if their paths are identical then they are ordered according to
     *   the ordering of their queries; if the queries are identical then they
     *   are ordered according to the order of their fragments. </p></li>
     *
     * </ul>
     *
     * <p> This method satisfies the general contract of the {@link
     * java.lang.Comparable#compareTo(Object) Comparable.compareTo}
     * method. </p>
     *
     * @param   that
     *          The object to which this URI is to be compared
     *
     * @return  A negative integer, zero, or a positive integer as this URI is
     *          less than, equal to, or greater than the given URI
     *
     * @throws  ClassCastException
     *          If the given object is not a URI
     */
    public int compareTo(URI that) {
	int c;

	if ((c = compareIgnoringCase(this.scheme, that.scheme)) != 0)
	    return c;

	if (this.isOpaque()) {
	    if (that.isOpaque()) {
		// Both opaque
		if ((c = compare(this.schemeSpecificPart,
				 that.schemeSpecificPart)) != 0)
		    return c;
		return compare(this.fragment, that.fragment);
	    }
	    return +1;			// Opaque > hierarchical
	} else if (that.isOpaque()) {
	    return -1;			// Hierarchical < opaque
	}

	// Hierarchical
	if ((this.host != null) && (that.host != null)) {
	    // Both server-based
	    if ((c = compare(this.userInfo, that.userInfo)) != 0)
		return c;
	    if ((c = compareIgnoringCase(this.host, that.host)) != 0)
		return c;
	    if ((c = this.port - that.port) != 0)
		return c;
	} else {
	    // If one or both authorities are registry-based then we simply
	    // compare them in the usual, case-sensitive way.  If one is
	    // registry-based and one is server-based then the strings are
	    // guaranteed to be unequal, hence the comparison will never return
	    // zero and the compareTo and equals methods will remain
	    // consistent.
	    if ((c = compare(this.authority, that.authority)) != 0) return c;
	}

	if ((c = compare(this.path, that.path)) != 0) return c;
	if ((c = compare(this.query, that.query)) != 0) return c;
	return compare(this.fragment, that.fragment);
    }

    /**
     * Returns the content of this URI as a string.
     *
     * <p> If this URI was created by invoking one of the constructors in this
     * class then a string equivalent to the original input string, or to the
     * string computed from the originally-given components, as appropriate, is
     * returned.  Otherwise this URI was created by normalization, resolution,
     * or relativization, and so a string is constructed from this URI's
     * components according to the rules specified in <a
     * href="http://www.ietf.org/rfc/rfc2396.txt">RFC&nbsp;2396</a>,
     * section&nbsp;5.2, step&nbsp;7. </p>
     *
     * @return  The string form of this URI
     */
    public String toString() {
	defineString();
	return string;
    }

    /**
     * Returns the content of this URI as a US-ASCII string.
     *
     * <p> If this URI does not contain any characters in the <i>other</i>
     * category then an invocation of this method will return the same value as
     * an invocation of the {@link #toString() toString} method.  Otherwise
     * this method works as if by invoking that method and then <a
     * href="#encode">encoding</a> the result.  </p>
     *
     * @return  The string form of this URI, encoded as needed
     *          so that it only contains characters in the US-ASCII
     *          charset
     */
    public String toASCIIString() {
	defineString();
	return encode(string);
    }


    // -- Serialization support --

    /**
     * Saves the content of this URI to the given serial stream.
     *
     * <p> The only serializable field of a URI instance is its <tt>string</tt>
     * field.  That field is given a value, if it does not have one already,
     * and then the {@link java.io.ObjectOutputStream#defaultWriteObject()}
     * method of the given object-output stream is invoked. </p>
     *
     * @param  os  The object-output stream to which this object
     *             is to be written
     */
    private void writeObject(ObjectOutputStream os)
	throws IOException
    {
	defineString();
	os.defaultWriteObject();	// Writes the string field only
    }

    /**
     * Reconstitutes a URI from the given serial stream.
     *
     * <p> The {@link java.io.ObjectInputStream#defaultReadObject()} method is
     * invoked to read the value of the <tt>string</tt> field.  The result is
     * then parsed in the usual way.
     *
     * @param  is  The object-input stream from which this object
     *             is being read
     */
    private void readObject(ObjectInputStream is)
	throws ClassNotFoundException, IOException
    {
	port = -1;			// Argh
	is.defaultReadObject();
	try {
	    new Parser(string).parse(false);
	} catch (URISyntaxException x) {
	    IOException y = new InvalidObjectException("Invalid URI");
	    y.initCause(x);
	    throw y;
	}
    }


    // -- End of public methods --


    // -- Utility methods for string-field comparison and hashing --

    // These methods return appropriate values for null string arguments,
    // thereby simplifying the equals, hashCode, and compareTo methods.
    //
    // The case-ignoring methods should only be applied to strings whose
    // characters are all known to be US-ASCII.  Because of this restriction,
    // these methods are faster than the similar methods in the String class.

    // US-ASCII only
    private static int toLower(char c) {
	if ((c >= 'A') && (c <= 'Z'))
	    return c + ('a' - 'A');
	return c;
    }

    private static boolean equal(String s, String t) {
	if (s == t) return true;
	if ((s != null) && (t != null)) {
	    if (s.length() != t.length())
		return false;
	    if (s.indexOf('%') < 0)
		return s.equals(t);
	    int n = s.length();
	    for (int i = 0; i < n;) {
		char c = s.charAt(i);
		char d = t.charAt(i);
		if (c != '%') {
		    if (c != d)
			return false;
		    i++;
		    continue;
		}
		i++;
		if (toLower(s.charAt(i)) != toLower(t.charAt(i)))
		    return false;
		i++;
		if (toLower(s.charAt(i)) != toLower(t.charAt(i)))
		    return false;
		i++;
	    }
	    return true;
	}
	return false;
    }

    // US-ASCII only
    private static boolean equalIgnoringCase(String s, String t) {
	if (s == t) return true;
	if ((s != null) && (t != null)) {
	    int n = s.length();
	    if (t.length() != n)
		return false;
	    for (int i = 0; i < n; i++) {
		if (toLower(s.charAt(i)) != toLower(t.charAt(i)))
		    return false;
	    }
	    return true;
	}
	return false;
    }

    private static int hash(int hash, String s) {
	if (s == null) return hash;
	return hash * 127 + s.hashCode();
    }

    // US-ASCII only
    private static int hashIgnoringCase(int hash, String s) {
	if (s == null) return hash;
	int h = hash;
	int n = s.length();
	for (int i = 0; i < n; i++)
	    h = 31 * h + toLower(s.charAt(i));
	return h;
    }

    private static int compare(String s, String t) {
	if (s == t) return 0;
	if (s != null) {
	    if (t != null)
		return s.compareTo(t);
	    else
		return +1;
	} else {
	    return -1;
	}
    }

    // US-ASCII only
    private static int compareIgnoringCase(String s, String t) {
	if (s == t) return 0;
	if (s != null) {
	    if (t != null) {
		int sn = s.length();
		int tn = t.length();
		int n = sn < tn ? sn : tn;
		for (int i = 0; i < n; i++) {
		    int c = toLower(s.charAt(i)) - toLower(t.charAt(i));
		    if (c != 0)
			return c;
		}
		return sn - tn;
	    }
	    return +1;
	} else {
	    return -1;
	}
    }


    // -- String construction --

    // If a scheme is given then the path, if given, must be absolute
    //
    private static void checkPath(String s, String scheme, String path)
	throws URISyntaxException
    {
	if (scheme != null) {
	    if ((path != null)
		&& ((path.length() > 0) && (path.charAt(0) != '/')))
		throw new URISyntaxException(s,
					     "Relative path in absolute URI");
	}
    }

    private void appendAuthority(StringBuffer sb,
				 String authority,
				 String userInfo,
				 String host,
				 int port)
    {
	if (host != null) {
	    sb.append("//");
	    if (userInfo != null) {
		sb.append(quote(userInfo, L_USERINFO, H_USERINFO));
		sb.append('@');
	    }
	    boolean needBrackets = ((host.indexOf(':') >= 0)
				    && !host.startsWith("[")
				    && !host.endsWith("]"));
	    if (needBrackets) sb.append('[');
	    sb.append(host);
	    if (needBrackets) sb.append(']');
	    if (port != -1) {
		sb.append(':');
		sb.append(port);
	    }
	} else if (authority != null) {
	    sb.append("//");
	    if (authority.startsWith("[")) {
		int end = authority.indexOf("]");
		if (end != -1 && authority.indexOf(":")!=-1) {
		    String doquote, dontquote;
		    if (end == authority.length()) {
			dontquote = authority;
			doquote = "";
		    } else {
		    	dontquote = authority.substring(0,end+1);
			doquote = authority.substring(end+1);
		    }
		    sb.append (dontquote);
	    	    sb.append(quote(doquote, 
			    L_REG_NAME | L_SERVER,
			    H_REG_NAME | H_SERVER));
		}
	    } else {
	    	sb.append(quote(authority,
			    L_REG_NAME | L_SERVER,
			    H_REG_NAME | H_SERVER));
	    }
	}
    }

    private void appendSchemeSpecificPart(StringBuffer sb,
					  String opaquePart,
					  String authority,
					  String userInfo,
					  String host,
					  int port,
					  String path,
					  String query)
    {
	if (opaquePart != null) {
	    /* check if SSP begins with an IPv6 address
	     * because we must not quote a literal IPv6 address
	     */
	    if (opaquePart.startsWith("//[")) {
		int end =  opaquePart.indexOf("]");
		if (end != -1 && opaquePart.indexOf(":")!=-1) {
		    String doquote, dontquote;
		    if (end == opaquePart.length()) {
			dontquote = opaquePart;
			doquote = "";
		    } else {
		    	dontquote = opaquePart.substring(0,end+1);
			doquote = opaquePart.substring(end+1);
		    }
		    sb.append (dontquote);
	    	    sb.append(quote(doquote, L_URIC, H_URIC));
		}
	    } else {
	    	sb.append(quote(opaquePart, L_URIC, H_URIC));
	    }
	} else {
	    appendAuthority(sb, authority, userInfo, host, port);
	    if (path != null) 
		sb.append(quote(path, L_PATH, H_PATH));
	    if (query != null) {
		sb.append('?');
		sb.append(quote(query, L_URIC, H_URIC));
	    }
	}
    }

    private void appendFragment(StringBuffer sb, String fragment) {
	if (fragment != null) {
	    sb.append('#');
	    sb.append(quote(fragment, L_URIC, H_URIC));
	}
    }

    private String toString(String scheme,
			    String opaquePart,
			    String authority,
			    String userInfo,
			    String host,
			    int port,
			    String path,
			    String query,
			    String fragment)
    {
	StringBuffer sb = new StringBuffer();
	if (scheme != null) {
	    sb.append(scheme);
	    sb.append(':');
	}
	appendSchemeSpecificPart(sb, opaquePart,
				 authority, userInfo, host, port,
				 path, query);
	appendFragment(sb, fragment);
	return sb.toString();
    }

    private void defineSchemeSpecificPart() {
	if (schemeSpecificPart != null) return;
	StringBuffer sb = new StringBuffer();
	appendSchemeSpecificPart(sb, null, getAuthority(), getUserInfo(),
				 host, port, getPath(), getQuery());
	if (sb.length() == 0) return;
	schemeSpecificPart = sb.toString();
    }

    private void defineString() {
	if (string != null) return;

	StringBuffer sb = new StringBuffer();
        if (scheme != null) {
            sb.append(scheme);
            sb.append(':');
        }
	if (isOpaque()) {
            sb.append(schemeSpecificPart);
        } else {
	    if (host != null) {
                sb.append("//");
                if (userInfo != null) {
                    sb.append(userInfo);
                    sb.append('@');
                }
                boolean needBrackets = ((host.indexOf(':') >= 0)
                                    && !host.startsWith("[")
                                    && !host.endsWith("]"));
                if (needBrackets) sb.append('[');
                sb.append(host);
                if (needBrackets) sb.append(']');
                if (port != -1) {
                    sb.append(':');
                    sb.append(port);
                }
            } else if (authority != null) {
                sb.append("//");
                sb.append(authority);
	    }
            if (path != null)
                sb.append(path);
            if (query != null) {
                sb.append('?');
                sb.append(query);
            }
        }
	if (fragment != null) {
            sb.append('#');
            sb.append(fragment);
	}
	string = sb.toString();
    }


    // -- Normalization, resolution, and relativization --

    // RFC2396 5.2 (6)
    private static String resolvePath(String base, String child,
				      boolean absolute)
    {
        int i = base.lastIndexOf('/');
	int cn = child.length();
	String path = "";

	if (cn == 0) {
	    // 5.2 (6a)
	    if (i >= 0)
		path = base.substring(0, i + 1);
	} else {
	    StringBuffer sb = new StringBuffer(base.length() + cn);
	    // 5.2 (6a)
	    if (i >= 0)
		sb.append(base.substring(0, i + 1));
	    // 5.2 (6b)
	    sb.append(child);
	    path = sb.toString();
	}

	// 5.2 (6c-f)
	String np = normalize(path);

	// 5.2 (6g): If the result is absolute but the path begins with "../",
	// then we simply leave the path as-is

	return np;
    }

    // RFC2396 5.2
    private static URI resolve(URI base, URI child) {
	// check if child if opaque first so that NPE is thrown 
	// if child is null.
	if (child.isOpaque() || base.isOpaque())
	    return child;

	// 5.2 (2): Reference to current document (lone fragment)
	if ((child.scheme == null) && (child.authority == null)
	    && child.path.equals("") && (child.fragment != null)
	    && (child.query == null)) {
	    if ((base.fragment != null)
		&& child.fragment.equals(base.fragment)) {
		return base;
	    }
	    URI ru = new URI();
	    ru.scheme = base.scheme;
	    ru.authority = base.authority;
	    ru.userInfo = base.userInfo;
	    ru.host = base.host;
	    ru.port = base.port;
	    ru.path = base.path;
	    ru.fragment = child.fragment;
	    ru.query = base.query;
	    return ru;
	}

	// 5.2 (3): Child is absolute
	if (child.scheme != null)
	    return child;

	URI ru = new URI();		// Resolved URI
	ru.scheme = base.scheme;
	ru.query = child.query;
	ru.fragment = child.fragment;

	// 5.2 (4): Authority
	if (child.authority == null) {
	    ru.authority = base.authority;
	    ru.host = base.host;
	    ru.userInfo = base.userInfo;
	    ru.port = base.port;

	    String cp = (child.path == null) ? "" : child.path;
	    if ((cp.length() > 0) && (cp.charAt(0) == '/')) {
		// 5.2 (5): Child path is absolute
		ru.path = child.path;
	    } else {
		// 5.2 (6): Resolve relative path
		ru.path = resolvePath(base.path, cp, base.isAbsolute());
	    }
	} else {
	    ru.authority = child.authority;
	    ru.host = child.host;
	    ru.userInfo = child.userInfo;
	    ru.host = child.host;
	    ru.port = child.port;
	    ru.path = child.path;
	}

	// 5.2 (7): Recombine (nothing to do here)
	return ru;
    }

    // If the given URI's path is normal then return the URI;
    // o.w., return a new URI containing the normalized path.
    //
    private static URI normalize(URI u) {
	if (u.isOpaque() || (u.path == null) || (u.path.length() == 0))
	    return u;

	String np = normalize(u.path);
	if (np == u.path)
	    return u;

	URI v = new URI();
	v.scheme = u.scheme;
	v.fragment = u.fragment;
	v.authority = u.authority;
	v.userInfo = u.userInfo;
	v.host = u.host;
	v.port = u.port;
	v.path = np;
	v.query = u.query;
	return v;
    }

    // If both URIs are hierarchical, their scheme and authority components are
    // identical, and the base path is a prefix of the child's path, then
    // return a relative URI that, when resolved against the base, yields the
    // child; otherwise, return the child.
    //
    private static URI relativize(URI base, URI child) {
	// check if child if opaque first so that NPE is thrown 
        // if child is null.
	if (child.isOpaque() || base.isOpaque())
	    return child;
	if (!equalIgnoringCase(base.scheme, child.scheme)
	    || !equal(base.authority, child.authority))
	    return child;

	String bp = normalize(base.path);
	String cp = normalize(child.path);
	if (!bp.equals(cp)) {
	    if (!bp.endsWith("/"))
		bp = bp + "/";
	    if (!cp.startsWith(bp))
		return child;
	}

	URI v = new URI();
	v.path = cp.substring(bp.length());
	v.query = child.query;
	v.fragment = child.fragment;
	return v;
    }



    // -- Path normalization --

    // The following algorithm for path normalization avoids the creation of a
    // string object for each segment, as well as the use of a string buffer to
    // compute the final result, by using a single char array and editing it in
    // place.  The array is first split into segments, replacing each slash
    // with '\0' and creating a segment-index array, each element of which is
    // the index of the first char in the corresponding segment.  We then walk
    // through both arrays, removing ".", "..", and other segments as necessary
    // by setting their entries in the index array to -1.  Finally, the two
    // arrays are used to rejoin the segments and compute the final result.
    //
    // This code is based upon src/solaris/native/java/io/canonicalize_md.c


    // Check the given path to see if it might need normalization.  A path
    // might need normalization if it contains duplicate slashes, a "."
    // segment, or a ".." segment.  Return -1 if no further normalization is
    // possible, otherwise return the number of segments found.
    //
    // This method takes a string argument rather than a char array so that
    // this test can be performed without invoking path.toCharArray().
    //
    static private int needsNormalization(String path) {
	boolean normal = true;
	int ns = 0;			// Number of segments
	int end = path.length() - 1;	// Index of last char in path
	int p = 0;			// Index of next char in path

	// Skip initial slashes
	while (p <= end) {
	    if (path.charAt(p) != '/') break;
	    p++;
	}
	if (p > 1) normal = false;

	// Scan segments
	while (p <= end) {

	    // Looking at "." or ".." ?
	    if ((path.charAt(p) == '.')
		&& ((p == end)
		    || ((path.charAt(p + 1) == '/')
			|| ((path.charAt(p + 1) == '.')
			    && ((p + 1 == end)
				|| (path.charAt(p + 2) == '/')))))) {
		normal = false;
	    }
	    ns++;

	    // Find beginning of next segment
	    while (p <= end) {
		if (path.charAt(p++) != '/')
		    continue;

		// Skip redundant slashes
		while (p <= end) {
		    if (path.charAt(p) != '/') break;
		    normal = false;
		    p++;
		}

		break;
	    }
	}

	return normal ? -1 : ns;
    }


    // Split the given path into segments, replacing slashes with nulls and
    // filling in the given segment-index array.
    //
    // Preconditions:
    //   segs.length == Number of segments in path
    //
    // Postconditions:
    //   All slashes in path replaced by '\0'
    //   segs[i] == Index of first char in segment i (0 <= i < segs.length)
    //
    static private void split(char[] path, int[] segs) {
	int end = path.length - 1;	// Index of last char in path
	int p = 0;			// Index of next char in path
	int i = 0;			// Index of current segment

	// Skip initial slashes
	while (p <= end) {
	    if (path[p] != '/') break;
	    path[p] = '\0';
	    p++;
	}

	while (p <= end) {

	    // Note start of segment
	    segs[i++] = p++;

	    // Find beginning of next segment
	    while (p <= end) {
		if (path[p++] != '/')
		    continue;
		path[p - 1] = '\0';

		// Skip redundant slashes
		while (p <= end) {
		    if (path[p] != '/') break;
		    path[p++] = '\0';
		}
		break;
	    }
	}

	if (i != segs.length)
	    throw new InternalError();	// ASSERT
    }


    // Join the segments in the given path according to the given segment-index
    // array, ignoring those segments whose index entries have been set to -1,
    // and inserting slashes as needed.  Return the length of the resulting
    // path.
    //
    // Preconditions:
    //   segs[i] == -1 implies segment i is to be ignored
    //   path computed by split, as above, with '\0' having replaced '/'
    //
    // Postconditions:
    //   path[0] .. path[return value] == Resulting path
    //
    static private int join(char[] path, int[] segs) {
	int ns = segs.length;		// Number of segments
	int end = path.length - 1;	// Index of last char in path
	int p = 0;			// Index of next path char to write

	if (path[p] == '\0') {
	    // Restore initial slash for absolute paths
	    path[p++] = '/';
	}

	for (int i = 0; i < ns; i++) {
	    int q = segs[i];		// Current segment
	    if (q == -1)
		// Ignore this segment
		continue;

	    if (p == q) {
		// We're already at this segment, so just skip to its end
		while ((p <= end) && (path[p] != '\0'))
		    p++;
		if (p <= end) {
		    // Preserve trailing slash
		    path[p++] = '/';
		}
	    } else if (p < q) {
		// Copy q down to p
		while ((q <= end) && (path[q] != '\0'))
		    path[p++] = path[q++];
		if (q <= end) {
		    // Preserve trailing slash
		    path[p++] = '/';
		}
	    } else
		throw new InternalError(); // ASSERT false
	}

	return p;
    }


    // Remove "." segments from the given path, and remove segment pairs
    // consisting of a non-".." segment followed by a ".." segment.
    //
    private static void removeDots(char[] path, int[] segs) {
	int ns = segs.length;
	int end = path.length - 1;

	for (int i = 0; i < ns; i++) {
	    int dots = 0;		// Number of dots found (0, 1, or 2)

	    // Find next occurrence of "." or ".."
	    do {
		int p = segs[i];
		if (path[p] == '.') {
		    if (p == end) {
			dots = 1;
			break;
		    } else if (path[p + 1] == '\0') {
			dots = 1;
			break;
		    } else if ((path[p + 1] == '.')
			       && ((p + 1 == end)
				   || (path[p + 2] == '\0'))) {
			dots = 2;
			break;
		    }
		}
		i++;
	    } while (i < ns);
	    if ((i > ns) || (dots == 0))
		break;

	    if (dots == 1) {
		// Remove this occurrence of "."
		segs[i] = -1;
	    } else {
		// If there is a preceding non-".." segment, remove both that
		// segment and this occurrence of ".."; otherwise, leave this
		// ".." segment as-is.
		int j;
		for (j = i - 1; j >= 0; j--) {
		    if (segs[j] != -1) break;
		}
		if (j >= 0) {
		    int q = segs[j];
		    if (!((path[q] == '.')
			  && (path[q + 1] == '.')
			  && (path[q + 2] == '\0'))) {
			segs[i] = -1;
			segs[j] = -1;
		    }
		}
	    }
	}
    }


    // DEVIATION: If the normalized path is relative, and if the first
    // segment could be parsed as a scheme name, then prepend a "." segment
    //
    private static void maybeAddLeadingDot(char[] path, int[] segs) {

	if (path[0] == '\0')
	    // The path is absolute
	    return;

	int ns = segs.length;
	int f = 0;			// Index of first segment
	while (f < ns) {
	    if (segs[f] >= 0)
		break;
	    f++;
	}
	if ((f >= ns) || (f == 0))
	    // The path is empty, or else the original first segment survived,
	    // in which case we already know that no leading "." is needed
	    return;

	int p = segs[f];
	while ((p < path.length) && (path[p] != ':') && (path[p] != '\0')) p++;
	if (p >= path.length || path[p] == '\0')
	    // No colon in first segment, so no "." needed
	    return;

	// At this point we know that the first segment is unused,
	// hence we can insert a "." segment at that position
	path[0] = '.';
	path[1] = '\0';
	segs[0] = 0;
    }


    // Normalize the given path string.  A normal path string has no empty
    // segments (i.e., occurrences of "//"), no segments equal to ".", and no
    // segments equal to ".." that are preceded by a segment not equal to "..".
    // In contrast to Unix-style pathname normalization, for URI paths we
    // always retain trailing slashes.
    //
    private static String normalize(String ps) {

	// Does this path need normalization?
	int ns = needsNormalization(ps);	// Number of segments
	if (ns < 0)
	    // Nope -- just return it
	    return ps;

	char[] path = ps.toCharArray();		// Path in char-array form

	// Split path into segments
	int[] segs = new int[ns];		// Segment-index array
	split(path, segs);

	// Remove dots
	removeDots(path, segs);

	// Prevent scheme-name confusion
	maybeAddLeadingDot(path, segs);

	// Join the remaining segments and return the result
	String s = new String(path, 0, join(path, segs));
	if (s.equals(ps)) {
	    // string was already normalized
	    return ps;
	}
	return s;
    }



    // -- Character classes for parsing --

    // RFC2396 precisely specifies which characters in the US-ASCII charset are
    // permissible in the various components of a URI reference.  We here
    // define a set of mask pairs to aid in enforcing these restrictions.  Each
    // mask pair consists of two longs, a low mask and a high mask.  Taken
    // together they represent a 128-bit mask, where bit i is set iff the
    // character with value i is permitted.
    //
    // This approach is more efficient than sequentially searching arrays of
    // permitted characters.  It could be made still more efficient by
    // precompiling the mask information so that a character's presence in a
    // given mask could be determined by a single table lookup.

    // Compute the low-order mask for the characters in the given string
    private static long lowMask(String chars) {
	int n = chars.length();
	long m = 0;
	for (int i = 0; i < n; i++) {
	    char c = chars.charAt(i);
	    if (c < 64)
		m |= (1L << c);
	}
	return m;
    }

    // Compute the high-order mask for the characters in the given string
    private static long highMask(String chars) {
	int n = chars.length();
	long m = 0;
	for (int i = 0; i < n; i++) {
	    char c = chars.charAt(i);
	    if ((c >= 64) && (c < 128))
		m |= (1L << (c - 64));
	}
	return m;
    }

    // Compute a low-order mask for the characters
    // between first and last, inclusive
    private static long lowMask(char first, char last) {
	long m = 0;
	int f = Math.max(Math.min(first, 63), 0);
	int l = Math.max(Math.min(last, 63), 0);
	for (int i = f; i <= l; i++)
	    m |= 1L << i;
	return m;
    }

    // Compute a high-order mask for the characters
    // between first and last, inclusive
    private static long highMask(char first, char last) {
	long m = 0;
	int f = Math.max(Math.min(first, 127), 64) - 64;
	int l = Math.max(Math.min(last, 127), 64) - 64;
	for (int i = f; i <= l; i++)
	    m |= 1L << i;
	return m;
    }

    // Tell whether the given character is permitted by the given mask pair
    private static boolean match(char c, long lowMask, long highMask) {
	if (c < 64)
	    return ((1L << c) & lowMask) != 0;
	if (c < 128)
	    return ((1L << (c - 64)) & highMask) != 0;
	return false;
    }

    // Character-class masks, in reverse order from RFC2396 because
    // initializers for static fields cannot make forward references.

    // digit    = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" |
    //            "8" | "9"
    private static final long L_DIGIT = lowMask('0', '9');
    private static final long H_DIGIT = 0L;

    // upalpha  = "A" | "B" | "C" | "D" | "E" | "F" | "G" | "H" | "I" |
    //            "J" | "K" | "L" | "M" | "N" | "O" | "P" | "Q" | "R" |
    //            "S" | "T" | "U" | "V" | "W" | "X" | "Y" | "Z"
    private static final long L_UPALPHA = 0L;
    private static final long H_UPALPHA = highMask('A', 'Z');

    // lowalpha = "a" | "b" | "c" | "d" | "e" | "f" | "g" | "h" | "i" |
    //            "j" | "k" | "l" | "m" | "n" | "o" | "p" | "q" | "r" |
    //            "s" | "t" | "u" | "v" | "w" | "x" | "y" | "z"
    private static final long L_LOWALPHA = 0L;
    private static final long H_LOWALPHA = highMask('a', 'z');

    // alpha         = lowalpha | upalpha
    private static final long L_ALPHA = L_LOWALPHA | L_UPALPHA;
    private static final long H_ALPHA = H_LOWALPHA | H_UPALPHA;

    // alphanum      = alpha | digit
    private static final long L_ALPHANUM = L_DIGIT | L_ALPHA;
    private static final long H_ALPHANUM = H_DIGIT | H_ALPHA;

    // hex           = digit | "A" | "B" | "C" | "D" | "E" | "F" |
    //                         "a" | "b" | "c" | "d" | "e" | "f"
    private static final long L_HEX = L_DIGIT;
    private static final long H_HEX = highMask('A', 'F') | highMask('a', 'f');

    // mark          = "-" | "_" | "." | "!" | "~" | "*" | "'" |
    //                 "(" | ")"
    private static final long L_MARK = lowMask("-_.!~*'()");
    private static final long H_MARK = highMask("-_.!~*'()");

    // unreserved    = alphanum | mark
    private static final long L_UNRESERVED = L_ALPHANUM | L_MARK;
    private static final long H_UNRESERVED = H_ALPHANUM | H_MARK;

    // reserved      = ";" | "/" | "?" | ":" | "@" | "&" | "=" | "+" |
    //                 "$" | "," | "[" | "]"
    // Added per RFC2732: "[", "]"
    private static final long L_RESERVED = lowMask(";/?:@&=+$,[]");
    private static final long H_RESERVED = highMask(";/?:@&=+$,[]");

    // The zero'th bit is used to indicate that escape pairs and non-US-ASCII
    // characters are allowed; this is handled by the scanEscape method below.
    private static final long L_ESCAPED = 1L;
    private static final long H_ESCAPED = 0L;

    // uric          = reserved | unreserved | escaped
    private static final long L_URIC = L_RESERVED | L_UNRESERVED | L_ESCAPED;
    private static final long H_URIC = H_RESERVED | H_UNRESERVED | H_ESCAPED;

    // pchar         = unreserved | escaped |
    //                 ":" | "@" | "&" | "=" | "+" | "$" | ","
    private static final long L_PCHAR
	= L_UNRESERVED | L_ESCAPED | lowMask(":@&=+$,");
    private static final long H_PCHAR
	= H_UNRESERVED | H_ESCAPED | highMask(":@&=+$,");

    // All valid path characters
    private static final long L_PATH = L_PCHAR | lowMask(";/");
    private static final long H_PATH = H_PCHAR | highMask(";/");

    // Dash, for use in domainlabel and toplabel
    private static final long L_DASH = lowMask("-");
    private static final long H_DASH = highMask("-");

    // Dot, for use in hostnames
    private static final long L_DOT = lowMask(".");
    private static final long H_DOT = highMask(".");

    // userinfo      = *( unreserved | escaped |
    //                    ";" | ":" | "&" | "=" | "+" | "$" | "," )
    private static final long L_USERINFO
	= L_UNRESERVED | L_ESCAPED | lowMask(";:&=+$,");
    private static final long H_USERINFO
	= H_UNRESERVED | H_ESCAPED | highMask(";:&=+$,");

    // reg_name      = 1*( unreserved | escaped | "$" | "," |
    //                     ";" | ":" | "@" | "&" | "=" | "+" )
    private static final long L_REG_NAME
	= L_UNRESERVED | L_ESCAPED | lowMask("$,;:@&=+");
    private static final long H_REG_NAME
	= H_UNRESERVED | H_ESCAPED | highMask("$,;:@&=+");

    // All valid characters for server-based authorities
    private static final long L_SERVER
	= L_USERINFO | L_ALPHANUM | L_DASH | lowMask(".:@[]");
    private static final long H_SERVER
	= H_USERINFO | H_ALPHANUM | H_DASH | highMask(".:@[]");

    // Special case of server authority that represents an IPv6 address
    // In this case, a % does not signify an escape sequence
    private static final long L_SERVER_PERCENT
	= L_SERVER | lowMask("%");
    private static final long H_SERVER_PERCENT
	= H_SERVER | highMask("%");
    private static final long L_LEFT_BRACKET = lowMask("[");
    private static final long H_LEFT_BRACKET = highMask("[");

    // scheme        = alpha *( alpha | digit | "+" | "-" | "." )
    private static final long L_SCHEME = L_ALPHA | L_DIGIT | lowMask("+-.");
    private static final long H_SCHEME = H_ALPHA | H_DIGIT | highMask("+-.");

    // uric_no_slash = unreserved | escaped | ";" | "?" | ":" | "@" |
    //                 "&" | "=" | "+" | "$" | ","
    private static final long L_URIC_NO_SLASH
	= L_UNRESERVED | L_ESCAPED | lowMask(";?:@&=+$,");
    private static final long H_URIC_NO_SLASH
	= H_UNRESERVED | H_ESCAPED | highMask(";?:@&=+$,");


    // -- Escaping and encoding --

    private final static char[] hexDigits = {
	'0', '1', '2', '3', '4', '5', '6', '7',
	'8', '9', 'A', 'B', 'C', 'D', 'E', 'F'
    };

    private static void appendEscape(StringBuffer sb, byte b) {
	sb.append('%');
	sb.append(hexDigits[(b >> 4) & 0x0f]);
	sb.append(hexDigits[(b >> 0) & 0x0f]);
    }

    private static void appendEncoded(StringBuffer sb, char c) {
	ByteBuffer bb = null;
	try {
	    bb = ThreadLocalCoders.encoderFor("UTF-8")
		.encode(CharBuffer.wrap("" + c));
	} catch (CharacterCodingException x) {
	    assert false;
	}
	while (bb.hasRemaining()) {
	    int b = bb.get() & 0xff;
	    if (b >= 0x80)
		appendEscape(sb, (byte)b);
	    else
		sb.append((char)b);
	}
    }

    // Quote any characters in s that are not permitted
    // by the given mask pair
    //
    private static String quote(String s, long lowMask, long highMask) {
	int n = s.length();
	StringBuffer sb = null;
	boolean allowNonASCII = ((lowMask & L_ESCAPED) != 0);
	for (int i = 0; i < s.length(); i++) {
	    char c = s.charAt(i);
	    if (c < '\u0080') {
		if (!match(c, lowMask, highMask)) {
		    if (sb == null) {
			sb = new StringBuffer();
			sb.append(s.substring(0, i));
		    }
		    appendEscape(sb, (byte)c);
		} else {
		    if (sb != null)
			sb.append(c);
		}
	    } else if (allowNonASCII
		       && (Character.isSpaceChar(c)
			   || Character.isISOControl(c))) {
		if (sb == null) {
		    sb = new StringBuffer();
		    sb.append(s.substring(0, i));
		}
		appendEncoded(sb, c);
	    } else {
		if (sb != null)
		    sb.append(c);
	    }
	}
	return (sb == null) ? s : sb.toString();
    }

    // Encodes all characters >= \u0080 into escaped, normalized UTF-8 octets,
    // assuming that s is otherwise legal
    //
    private static String encode(String s) {
	int n = s.length();
	if (n == 0)
	    return s;

	// First check whether we actually need to encode
	for (int i = 0;;) {
	    if (s.charAt(i) >= '\u0080')
		break;
	    if (++i >= n)
		return s;
	}

	String ns = Normalizer.normalize(s, Normalizer.Form.NFC);
	ByteBuffer bb = null;
	try {
	    bb = ThreadLocalCoders.encoderFor("UTF-8")
		.encode(CharBuffer.wrap(ns));
	} catch (CharacterCodingException x) {
	    assert false;
	}

	StringBuffer sb = new StringBuffer();
	while (bb.hasRemaining()) {
	    int b = bb.get() & 0xff;
	    if (b >= 0x80)
		appendEscape(sb, (byte)b);
	    else
		sb.append((char)b);
	}
	return sb.toString();
    }

    private static int decode(char c) {
	if ((c >= '0') && (c <= '9'))
	    return c - '0';
	if ((c >= 'a') && (c <= 'f'))
	    return c - 'a' + 10;
	if ((c >= 'A') && (c <= 'F'))
	    return c - 'A' + 10;
	assert false;
	return -1;
    }

    private static byte decode(char c1, char c2) {
	return (byte)(  ((decode(c1) & 0xf) << 4)
		      | ((decode(c2) & 0xf) << 0));
    }

    // Evaluates all escapes in s, applying UTF-8 decoding if needed.  Assumes
    // that escapes are well-formed syntactically, i.e., of the form %XX.  If a
    // sequence of escaped octets is not valid UTF-8 then the erroneous octets
    // are replaced with '\uFFFD'.
    // Exception: any "%" found between "[]" is left alone. It is an IPv6 literal
    //            with a scope_id
    //
    private static String decode(String s) {
	if (s == null)
	    return s;
	int n = s.length();
	if (n == 0)
	    return s;
	if (s.indexOf('%') < 0)
	    return s;

	byte[] ba = new byte[n];
	StringBuffer sb = new StringBuffer(n);
	ByteBuffer bb = ByteBuffer.allocate(n);
	CharBuffer cb = CharBuffer.allocate(n);
	CharsetDecoder dec = ThreadLocalCoders.decoderFor("UTF-8")
	    .onMalformedInput(CodingErrorAction.REPLACE)
	    .onUnmappableCharacter(CodingErrorAction.REPLACE);

	// This is not horribly efficient, but it will do for now
	char c = s.charAt(0);
    	boolean betweenBrackets = false;

	for (int i = 0; i < n;) {
	    assert c == s.charAt(i);	// Loop invariant
	    if (c == '[') {
		betweenBrackets = true;
	    } else if (betweenBrackets && c == ']') {
		betweenBrackets = false;
	    }
	    if (c != '%' || betweenBrackets) {
		sb.append(c);
		if (++i >= n)
		    break;
		c = s.charAt(i);
		continue;
	    }
	    bb.clear();
	    int ui = i;
	    for (;;) {
		assert (n - i >= 2);
		bb.put(decode(s.charAt(++i), s.charAt(++i)));
		if (++i >= n)
		    break;
		c = s.charAt(i);
		if (c != '%')
		    break;
	    }
	    bb.flip();
	    cb.clear();
	    dec.reset();
	    CoderResult cr = dec.decode(bb, cb, true);
	    assert cr.isUnderflow();
	    cr = dec.flush(cb);
	    assert cr.isUnderflow();
	    sb.append(cb.flip().toString());
	}

	return sb.toString();
    }


    // -- Parsing --

    // For convenience we wrap the input URI string in a new instance of the
    // following internal class.  This saves always having to pass the input
    // string as an argument to each internal scan/parse method.

    private class Parser {

	private String input;		// URI input string
	private boolean requireServerAuthority = false;

	Parser(String s) {
	    input = s;
	    string = s;
	}

	// -- Methods for throwing URISyntaxException in various ways --

	private void fail(String reason) throws URISyntaxException {
	    throw new URISyntaxException(input, reason);
	}

	private void fail(String reason, int p) throws URISyntaxException {
	    throw new URISyntaxException(input, reason, p);
	}

	private void failExpecting(String expected, int p)
	    throws URISyntaxException
	{
	    fail("Expected " + expected, p);
	}

	private void failExpecting(String expected, String prior, int p)
	    throws URISyntaxException
	{
	    fail("Expected " + expected + " following " + prior, p);
	}


	// -- Simple access to the input string --

	// Return a substring of the input string
	//
	private String substring(int start, int end) {
	    return input.substring(start, end);
	}

	// Return the char at position p,
	// assuming that p < input.length()
	//
	private char charAt(int p) {
	    return input.charAt(p);
	}

	// Tells whether start < end and, if so, whether charAt(start) == c
	//
	private boolean at(int start, int end, char c) {
	    return (start < end) && (charAt(start) == c);
	}

	// Tells whether start + s.length() < end and, if so,
	// whether the chars at the start position match s exactly
	//
	private boolean at(int start, int end, String s) {
	    int p = start;
	    int sn = s.length();
	    if (sn > end - p)
		return false;
	    int i = 0;
	    while (i < sn) {
		if (charAt(p++) != s.charAt(i)) {
		    break;
		}
		i++;
	    }
	    return (i == sn);
	}


	// -- Scanning --

	// The various scan and parse methods that follow use a uniform
	// convention of taking the current start position and end index as
	// their first two arguments.  The start is inclusive while the end is
	// exclusive, just as in the String class, i.e., a start/end pair
	// denotes the left-open interval [start, end) of the input string.
	//
	// These methods never proceed past the end position.  They may return
	// -1 to indicate outright failure, but more often they simply return
	// the position of the first char after the last char scanned.  Thus
	// a typical idiom is
	//
	//     int p = start;
	//     int q = scan(p, end, ...);
	//     if (q > p)
	//         // We scanned something
	//         ...;
	//     else if (q == p)
	//         // We scanned nothing
	//         ...;
	//     else if (q == -1)
	//         // Something went wrong
	//         ...;


	// Scan a specific char: If the char at the given start position is
	// equal to c, return the index of the next char; otherwise, return the
	// start position.
	//
	private int scan(int start, int end, char c) {
	    if ((start < end) && (charAt(start) == c))
		return start + 1;
	    return start;
	}

	// Scan forward from the given start position.  Stop at the first char
	// in the err string (in which case -1 is returned), or the first char
	// in the stop string (in which case the index of the preceding char is
	// returned), or the end of the input string (in which case the length
	// of the input string is returned).  May return the start position if
	// nothing matches.
	//
	private int scan(int start, int end, String err, String stop) {
	    int p = start;
	    while (p < end) {
		char c = charAt(p);
		if (err.indexOf(c) >= 0)
		    return -1;
		if (stop.indexOf(c) >= 0)
		    break;
		p++;
	    }
	    return p;
	}

	// Scan a potential escape sequence, starting at the given position,
	// with the given first char (i.e., charAt(start) == c).
	//
	// This method assumes that if escapes are allowed then visible
	// non-US-ASCII chars are also allowed.
	//
	private int scanEscape(int start, int n, char first)
	    throws URISyntaxException
	{
	    int p = start;
	    char c = first;
	    if (c == '%') {
		// Process escape pair
		if ((p + 3 <= n)
		    && match(charAt(p + 1), L_HEX, H_HEX)
		    && match(charAt(p + 2), L_HEX, H_HEX)) {
		    return p + 3;
		}
		fail("Malformed escape pair", p);
	    } else if ((c > 128)
		       && !Character.isSpaceChar(c)
		       && !Character.isISOControl(c)) {
		// Allow unescaped but visible non-US-ASCII chars
		return p + 1;
	    }
	    return p;
	}

	// Scan chars that match the given mask pair
	//
	private int scan(int start, int n, long lowMask, long highMask)
	    throws URISyntaxException
	{
	    int p = start;
	    while (p < n) {
		char c = charAt(p);
		if (match(c, lowMask, highMask)) {
		    p++;
		    continue;
		}
		if ((lowMask & L_ESCAPED) != 0) {
		    int q = scanEscape(p, n, c);
		    if (q > p) {
			p = q;
			continue;
		    }
		}
		break;
	    }
	    return p;
	}

	// Check that each of the chars in [start, end) matches the given mask
	//
	private void checkChars(int start, int end,
				long lowMask, long highMask,
				String what)
	    throws URISyntaxException
	{
	    int p = scan(start, end, lowMask, highMask);
	    if (p < end)
		fail("Illegal character in " + what, p);
	}

	// Check that the char at position p matches the given mask
	//
	private void checkChar(int p,
			       long lowMask, long highMask,
			       String what)
	    throws URISyntaxException
	{
	    checkChars(p, p + 1, lowMask, highMask, what);
	}


	// -- Parsing --

	// [<scheme>:]<scheme-specific-part>[#<fragment>]
	//
	void parse(boolean rsa) throws URISyntaxException {
	    requireServerAuthority = rsa;
	    int ssp;			// Start of scheme-specific part
	    int n = input.length();
	    int p = scan(0, n, "/?#", ":");
	    if ((p >= 0) && at(p, n, ':')) {
		if (p == 0)
		    failExpecting("scheme name", 0);
		checkChar(0, L_ALPHA, H_ALPHA, "scheme name");
		checkChars(1, p, L_SCHEME, H_SCHEME, "scheme name");
		scheme = substring(0, p);
		p++;			// Skip ':'
		ssp = p;
		if (at(p, n, '/')) {
		    p = parseHierarchical(p, n);
		} else {
		    int q = scan(p, n, "", "#");
		    if (q <= p)
			failExpecting("scheme-specific part", p);
		    checkChars(p, q, L_URIC, H_URIC, "opaque part");
		    p = q;
		}
	    } else {
		ssp = 0;
		p = parseHierarchical(0, n);
	    }
	    schemeSpecificPart = substring(ssp, p);
	    if (at(p, n, '#')) {
		checkChars(p + 1, n, L_URIC, H_URIC, "fragment");
		fragment = substring(p + 1, n);
		p = n;
	    }
	    if (p < n)
		fail("end of URI", p);
	}

	// [//authority]<path>[?<query>]
	//
	// DEVIATION from RFC2396: We allow an empty authority component as
	// long as it's followed by a non-empty path, query component, or
	// fragment component.  This is so that URIs such as "file:///foo/bar"
	// will parse.  This seems to be the intent of RFC2396, though the
	// grammar does not permit it.  If the authority is empty then the
	// userInfo, host, and port components are undefined.
	//
	// DEVIATION from RFC2396: We allow empty relative paths.  This seems
	// to be the intent of RFC2396, but the grammar does not permit it.
	// The primary consequence of this deviation is that "#f" parses as a
	// relative URI with an empty path.
	//
	private int parseHierarchical(int start, int n)
	    throws URISyntaxException
	{
	    int p = start;
	    if (at(p, n, '/') && at(p + 1, n, '/')) {
		p += 2;
		int q = scan(p, n, "", "/?#");
		if (q > p) {
		    p = parseAuthority(p, q);
		} else if (q < n) {
		    // DEVIATION: Allow empty authority prior to non-empty 
		    // path, query component or fragment identifier
		} else
		    failExpecting("authority", p);
	    }
	    int q = scan(p, n, "", "?#"); // DEVIATION: May be empty
	    checkChars(p, q, L_PATH, H_PATH, "path");
	    path = substring(p, q);
	    p = q;
	    if (at(p, n, '?')) {
		p++;
		q = scan(p, n, "", "#");
		checkChars(p, q, L_URIC, H_URIC, "query");
		query = substring(p, q);
		p = q;
	    }
	    return p;
	}

	// authority     = server | reg_name
	//
	// Ambiguity: An authority that is a registry name rather than a server
	// might have a prefix that parses as a server.  We use the fact that
	// the authority component is always followed by '/' or the end of the
	// input string to resolve this: If the complete authority did not
	// parse as a server then we try to parse it as a registry name.
	//
	private int parseAuthority(int start, int n)
	    throws URISyntaxException
	{
	    int p = start;
	    int q = p;
	    URISyntaxException ex = null;

	    boolean serverChars;
	    boolean regChars;

	    if (scan(p, n, "", "]") > p) {
		// contains a literal IPv6 address, therefore % is allowed
	    	serverChars = (scan(p, n, L_SERVER_PERCENT, H_SERVER_PERCENT) == n);
	    } else {
	    	serverChars = (scan(p, n, L_SERVER, H_SERVER) == n);
	    }
	    regChars = (scan(p, n, L_REG_NAME, H_REG_NAME) == n);

	    if (regChars && !serverChars) {
		// Must be a registry-based authority
		authority = substring(p, n);
		return n;
	    }

	    if (serverChars) {
		// Might be (probably is) a server-based authority, so attempt
		// to parse it as such.  If the attempt fails, try to treat it
		// as a registry-based authority.
		try {
		    q = parseServer(p, n);
		    if (q < n)
			failExpecting("end of authority", q);
		    authority = substring(p, n);
		} catch (URISyntaxException x) {
		    // Undo results of failed parse
		    userInfo = null;
		    host = null;
		    port = -1;
		    if (requireServerAuthority) {
			// If we're insisting upon a server-based authority,
			// then just re-throw the exception
			throw x;
		    } else {
			// Save the exception in case it doesn't parse as a
			// registry either
			ex = x;
			q = p;
		    }
		}
	    }

	    if (q < n) {
		if (regChars) {
		    // Registry-based authority
		    authority = substring(p, n);
		} else if (ex != null) {
		    // Re-throw exception; it was probably due to
		    // a malformed IPv6 address
		    throw ex;
		} else {
		    fail("Illegal character in authority", q);
		}
	    }

	    return n;
	}


	// [<userinfo>@]<host>[:<port>]
	//
	private int parseServer(int start, int n)
	    throws URISyntaxException
	{
	    int p = start;
	    int q;

	    // userinfo
	    q = scan(p, n, "/?#", "@");
	    if ((q >= p) && at(q, n, '@')) {
		checkChars(p, q, L_USERINFO, H_USERINFO, "user info");
		userInfo = substring(p, q);
		p = q + 1;		// Skip '@'
	    }

	    // hostname, IPv4 address, or IPv6 address
	    if (at(p, n, '[')) {
		// DEVIATION from RFC2396: Support IPv6 addresses, per RFC2732
		p++;
		q = scan(p, n, "/?#", "]");
		if ((q > p) && at(q, n, ']')) {
		    // look for a "%" scope id
		    int r = scan (p, q, "", "%");
		    if (r > p) {
		    	parseIPv6Reference(p, r);
			if (r+1 == q) {
			    fail ("scope id expected");
			}
			checkChars (r+1, q, L_ALPHANUM, H_ALPHANUM, 
						"scope id");
		    } else {
		    	parseIPv6Reference(p, q);
		    }
	    	    host = substring(p-1, q+1);
		    p = q + 1;
		} else {
		    failExpecting("closing bracket for IPv6 address", q);
		}
	    } else {
		q = parseIPv4Address(p, n);
		if (q <= p)
		    q = parseHostname(p, n);
		p = q;
	    }

	    // port
	    if (at(p, n, ':')) {
		p++;
		q = scan(p, n, "", "/");
		if (q > p) {
		    checkChars(p, q, L_DIGIT, H_DIGIT, "port number");
		    try {
			port = Integer.parseInt(substring(p, q));
		    } catch (NumberFormatException x) {
			fail("Malformed port number", p);
		    }
		    p = q;
		}
	    }
	    if (p < n)
		failExpecting("port number", p);

	    return p;
	}

	// Scan a string of decimal digits whose value fits in a byte
	//
	private int scanByte(int start, int n)
	    throws URISyntaxException
	{
	    int p = start;
	    int q = scan(p, n, L_DIGIT, H_DIGIT);
	    if (q <= p) return q;
	    if (Integer.parseInt(substring(p, q)) > 255) return p;
	    return q;
	}

	// Scan an IPv4 address.
	//
	// If the strict argument is true then we require that the given
	// interval contain nothing besides an IPv4 address; if it is false
	// then we only require that it start with an IPv4 address.
	//
	// If the interval does not contain or start with (depending upon the
	// strict argument) a legal IPv4 address characters then we return -1
	// immediately; otherwise we insist that these characters parse as a
	// legal IPv4 address and throw an exception on failure.
	//
	// We assume that any string of decimal digits and dots must be an IPv4
	// address.  It won't parse as a hostname anyway, so making that
	// assumption here allows more meaningful exceptions to be thrown.
	//
	private int scanIPv4Address(int start, int n, boolean strict)
	    throws URISyntaxException
	{
	    int p = start;
	    int q;
	    int m = scan(p, n, L_DIGIT | L_DOT, H_DIGIT | H_DOT);
	    if ((m <= p) || (strict && (m != n)))
		return -1;
	    for (;;) {
		// Per RFC2732: At most three digits per byte
		// Further constraint: Each element fits in a byte
		if ((q = scanByte(p, m)) <= p) break;   p = q;
		if ((q = scan(p, m, '.')) <= p) break;  p = q;
		if ((q = scanByte(p, m)) <= p) break;   p = q;
		if ((q = scan(p, m, '.')) <= p) break;  p = q;
		if ((q = scanByte(p, m)) <= p) break;   p = q;
		if ((q = scan(p, m, '.')) <= p) break;  p = q;
		if ((q = scanByte(p, m)) <= p) break;   p = q;
		if (q < m) break;
		return q;
	    }
	    fail("Malformed IPv4 address", q);
	    return -1;
	}

	// Take an IPv4 address: Throw an exception if the given interval
	// contains anything except an IPv4 address
	//
	private int takeIPv4Address(int start, int n, String expected)
	    throws URISyntaxException
	{
	    int p = scanIPv4Address(start, n, true);
	    if (p <= start)
		failExpecting(expected, start);
	    return p;
	}

	// Attempt to parse an IPv4 address, returning -1 on failure but
	// allowing the given interval to contain [:<characters>] after
	// the IPv4 address.
	//
	private int parseIPv4Address(int start, int n) {
	    int p;

	    try {
	        p = scanIPv4Address(start, n, false);
	    } catch (URISyntaxException x) {
		return -1;
            } catch (NumberFormatException nfe) {
		return -1;
            }

	    if (p > start && p < n) {
	        // IPv4 address is followed by something - check that
		// it's a ":" as this is the only valid character to
		// follow an address.
		if (charAt(p) != ':') {
		    p = -1;
		}
	    }

	    if (p > start)
		host = substring(start, p);

	    return p;
	}

	// hostname      = domainlabel [ "." ] | 1*( domainlabel "." ) toplabel [ "." ] 
	// domainlabel   = alphanum | alphanum *( alphanum | "-" ) alphanum
        // toplabel      = alpha | alpha *( alphanum | "-" ) alphanum
	//
	private int parseHostname(int start, int n)
	    throws URISyntaxException
	{
	    int p = start;
	    int q;
	    int l = -1;			// Start of last parsed label

	    do {
		// domainlabel = alphanum [ *( alphanum | "-" ) alphanum ]
		q = scan(p, n, L_ALPHANUM, H_ALPHANUM);
		if (q <= p)
		    break;
		l = p;
		if (q > p) {
		    p = q;
		    q = scan(p, n, L_ALPHANUM | L_DASH, H_ALPHANUM | H_DASH);
		    if (q > p) {
			if (charAt(q - 1) == '-')
			    fail("Illegal character in hostname", q - 1);
			p = q;
		    }
		}
		q = scan(p, n, '.');
		if (q <= p)
		    break;
		p = q;
	    } while (p < n);

	    if ((p < n) && !at(p, n, ':'))
		fail("Illegal character in hostname", p);

	    if (l < 0)
		failExpecting("hostname", start);

	    // for a fully qualified hostname check that the rightmost
	    // label starts with an alpha character.
	    if (l > start && !match(charAt(l), L_ALPHA, H_ALPHA)) {
		fail("Illegal character in hostname", l);
	    }

	    host = substring(start, p);
	    return p;
	}


	// IPv6 address parsing, from RFC2373: IPv6 Addressing Architecture
	//
	// Bug: The grammar in RFC2373 Appendix B does not allow addresses of
	// the form ::12.34.56.78, which are clearly shown in the examples
	// earlier in the document.  Here is the original grammar:
	//
	//   IPv6address = hexpart [ ":" IPv4address ]
	//   hexpart     = hexseq | hexseq "::" [ hexseq ] | "::" [ hexseq ]
	//   hexseq      = hex4 *( ":" hex4)
	//   hex4        = 1*4HEXDIG
	//
	// We therefore use the following revised grammar:
	//
	//   IPv6address = hexseq [ ":" IPv4address ]
	//                 | hexseq [ "::" [ hexpost ] ]
	//                 | "::" [ hexpost ]
	//   hexpost     = hexseq | hexseq ":" IPv4address | IPv4address
	//   hexseq      = hex4 *( ":" hex4)
	//   hex4        = 1*4HEXDIG
	//
	// This covers all and only the following cases:
	//
	//   hexseq
	//   hexseq : IPv4address
	//   hexseq ::
	//   hexseq :: hexseq
	//   hexseq :: hexseq : IPv4address
	//   hexseq :: IPv4address
	//   :: hexseq
	//   :: hexseq : IPv4address
	//   :: IPv4address
	//   ::
	//
	// Additionally we constrain the IPv6 address as follows :-
	//
	//  i.  IPv6 addresses without compressed zeros should contain
	//      exactly 16 bytes.
	//
	//  ii. IPv6 addresses with compressed zeros should contain
	//      less than 16 bytes.

	private int ipv6byteCount = 0;

	private int parseIPv6Reference(int start, int n)
	    throws URISyntaxException
	{
	    int p = start;
	    int q;
	    boolean compressedZeros = false;

	    q = scanHexSeq(p, n);

	    if (q > p) {
		p = q;
		if (at(p, n, "::")) {
		    compressedZeros = true;
		    p = scanHexPost(p + 2, n);
		} else if (at(p, n, ':')) {
		    p = takeIPv4Address(p + 1,  n, "IPv4 address");
		    ipv6byteCount += 4;
		}
	    } else if (at(p, n, "::")) {
		compressedZeros = true;
		p = scanHexPost(p + 2, n);
	    }
	    if (p < n)
		fail("Malformed IPv6 address", start);
	    if (ipv6byteCount > 16)
		fail("IPv6 address too long", start);
	    if (!compressedZeros && ipv6byteCount < 16) 
		fail("IPv6 address too short", start);
	    if (compressedZeros && ipv6byteCount == 16)
		fail("Malformed IPv6 address", start);

	    return p;
	}

	private int scanHexPost(int start, int n)
	    throws URISyntaxException
	{
	    int p = start;
	    int q;

	    if (p == n)
		return p;

	    q = scanHexSeq(p, n);
	    if (q > p) {
		p = q;
		if (at(p, n, ':')) {
		    p++;
		    p = takeIPv4Address(p, n, "hex digits or IPv4 address");
		    ipv6byteCount += 4;
		}
	    } else {
		p = takeIPv4Address(p, n, "hex digits or IPv4 address");
		ipv6byteCount += 4;
	    }
	    return p;
	}

	// Scan a hex sequence; return -1 if one could not be scanned
	//
	private int scanHexSeq(int start, int n)
	    throws URISyntaxException
	{
	    int p = start;
	    int q;

	    q = scan(p, n, L_HEX, H_HEX);
	    if (q <= p)
		return -1;
	    if (at(q, n, '.'))		// Beginning of IPv4 address
		return -1;
	    if (q > p + 4)
                fail("IPv6 hexadecimal digit sequence too long", p);
	    ipv6byteCount += 2;
	    p = q;
	    while (p < n) {
		if (!at(p, n, ':'))
		    break;
		if (at(p + 1, n, ':'))
		    break;		// "::"
		p++;
		q = scan(p, n, L_HEX, H_HEX);
		if (q <= p)
		    failExpecting("digits for an IPv6 address", p);
		if (at(q, n, '.')) {	// Beginning of IPv4 address
		    p--;
		    break;
		}
		if (q > p + 4)
		    fail("IPv6 hexadecimal digit sequence too long", p);
		ipv6byteCount += 2;
		p = q;
	    }

	    return p;
	}

    }

}
